Adds HDRI-based Image-Based Lighting (IBL) ambient term to the OpenGL rasterizer

[juanchuletas]

Pull Request Template

Description

Adds HDRI-based Image-Based Lighting (IBL) ambient term to the OpenGL rasterizer, plus a tunable flat ambient fallback for scenes with no HDRI loaded. Fixes the raster preview looking flat/dark and lit only by manually placed point lights.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Documentation (changes to docs only)
• Performance (improves performance)

Changes Made

• Added HDR (.hdr) equirectangular texture loading (Texture::genTextureHDR, GPUTexture::uploadFloat)
• Added IBLProbe (IBL/ibl_probe.hpp/.cpp): converts an equirectangular HDR image to a cubemap, then convolves it into a diffuse irradiance cubemap
• Added equirect-to-cubemap and irradiance convolution shaders (resources/equirect_to_cubemap.vs/.fs, resources/irradiance_convolve.vs/.fs)
• Added an equirect-sampling skybox shader pair (resources/equirect_skybox.vs/.fs) and CubeMap::buildHDR() to render an HDRI directly as a background without a full cubemap conversion
• Wired IBL into fungt_default.fs: new irradianceMap, hasIBL, iblIntensity uniforms; ambient term now samples real HDRI-derived irradiance instead of a flat 0.05 * material.ambient hack when an environment is loaded
• Added a tunable flat ambientColor uniform/default (SceneManager-owned) as the fallback ambient term when no HDRI environment is loaded
• Added Reinhard tonemap + gamma correction to fungt_default.fs's final output, since HDR irradiance/light values can exceed 1.0 and would otherwise hard-clip to white
• SceneManager::loadEnvironment(hdrPath) builds and owns an IBLProbe, binding its irradiance map and setting hasIBL/iblIntensity/ambientColor uniforms once per frame in renderScene()
• Extended GPUTexture/OpenGLTexture with allocateEmptyCubemap() for render-target cubemap allocation
• Refactored IBLProbe to use GPUBuffer (cube mesh) and GPUTexture/Texture (cubemap allocation) instead of raw OpenGL calls, consistent with the existing rendering abstraction layer; offscreen FBO/RBO calls for the two convolution passes remain raw OpenGL (no existing interface covers per-face cubemap render targets)
• Fixed an invalid OpenGL enum: OpenGLTexture::upload() set GL_TEXTURE_MAG_FILTER to a mipmap filter mode (only valid for MIN_FILTER) and never called glGenerateMipmap; corrected and added the missing mipmap generation call
• Fixed a texture-unit collision causing glValidateProgram failures: texture_diffuse1 (sampler2D) and irradianceMap (samplerCube) could both default to texture unit 0; both are now explicitly pinned to distinct units at shader link time
• Switched SimpleGeometry's Sphere and Plane primitives from the separate primitive_vs.glsl/primitive_fs.glsl shader pair to fungt_default.vs/.fs, so they pick up the same ambient/IBL/tonemap behavior as loaded models (vertex layouts were confirmed compatible)

Technical Details

• Irradiance convolution follows the standard hemisphere-integral approach (per-texel cosine-weighted sampling over the upper hemisphere), matching the widely used LearnOpenGL IBL reference implementation
• A real bug was found and fixed in the convolution shader: sampling the environment cubemap with implicit LOD (texture()) inside an offscreen FBO pass produced wildly inconsistent, sometimes blown-out brightness across cubemap faces, because automatic mip selection has no meaningful screen-space derivatives in that context; switched to explicit textureLod(..., 0.0)
• IBL intensity and flat ambient color are both runtime-tunable (SceneManager::setIBLIntensity, setAmbientColor) rather than hardcoded, since raw HDRI irradiance values are frequently too dim or too bright to use directly against arbitrary scene content
• Cube primitive path (resources/cube.vs/.fs) was intentionally left unchanged: it has no vertex normal data end-to-end (geometry generation never emits normals), so it cannot support the lighting model used by fungt_default.fs without additional geometry-side work; scoped out of this PR

Testing

• Tested locally
• Visual comparison before/after
• Performance benchmarked
• Edge cases verified

Verified via a dedicated test sample (Samples/hdr_test) against two HDRIs (a sunny daytime environment and a night environment) and multiple test models (an OBJ/MTL street asset, an FBX street asset). Confirmed via direct GPU texture readback (glGetTexImage) that the generated cubemap and irradiance map contain correct, non-garbage HDR data before and after the textureLod fix. Confirmed the flat-ambient fallback and IBL path both respond correctly to their respective intensity/color controls.

Screenshots / Results

Before	After
Flat, near-black ambient; only manually placed point lights contribute any light	Real ambient light derived from HDRI irradiance (or a tunable flat ambient), visible directional variation across surfaces

Performance Impact

• Render time before: not measured
• Render time after: not measured (irradiance/cubemap build is a one-time cost at loadEnvironment() call time, not per-frame)
• Memory usage: not measured

Related Issues

Closes #

Checklist

• Code compiles without warnings
• Code follows project style guidelines
• Self-reviewed my own code
• Commented hard-to-understand areas
• No unnecessary debug code left behind

Notes for Reviewers

• The cube primitive path (resources/cube.vs/.fs) still uses the old unlit/no-normal shader; fixing it requires adding normal generation to Geometries/cube.cpp first and is left as follow-up work
• Full PBR (metallic/roughness, Cook-Torrance) shading, prefiltered specular reflections, and a BRDF integration LUT were explicitly out of scope for this PR; only the diffuse/ambient IBL term was implemented on top of the existing Blinn-Phong shader
• Screen-space ambient occlusion (SSAO) does not exist in this codebase; flat/uniform ambient (with no HDRI loaded) will not produce shape-defining contact shadows on its own, this is expected and not a regression

[juanchuletas]

A common Scene using Models and geometries looks correct, according with the ambient light. To have more details on models with pure gray colors, without a specific light source on the scene, we need to add

SSAO ( Screen Space Ambient Occlusion)