Address thread-safety in lcms helpers

Add clarifying comments about concurrency and ThreadSanitizer findings in two lcms sources. In cmsplugin.c note that gmtime()'s shared static result is copied while holding the LCMS mutex to avoid TSan-reported races when creating ICC profiles concurrently. In cmswtpnt.c document that the D50 XYZ constant is safe for concurrent readers and that the xyY value is computed in a thread-confined way to avoid writing to a shared static on each call.

Author: bobsingor

third_party/lcms/src/cmsplugin.c

@@ -1057,6 +1057,9 @@ cmsBool _cmsGetTime(struct tm* ptr_time)
 
     _cmsEnterCriticalSectionPrimitive(&_cmsContextPoolHeadMutex);
     t = gmtime(&now);
+    // EmbedPDF: copy gmtime()'s shared static result while still holding the
+    // LCMS mutex. ThreadSanitizer flags copying it after unlock when ICC
+    // profiles are created concurrently.
     if (t != NULL)
         *ptr_time = *t;
     _cmsLeaveCriticalSectionPrimitive(&_cmsContextPoolHeadMutex);

third_party/lcms/src/cmswtpnt.c

@@ -30,13 +30,18 @@
 // D50 - Widely used
 const cmsCIEXYZ* CMSEXPORT cmsD50_XYZ(void)
 {
+    // EmbedPDF: immutable shared D50 constant is safe for concurrent readers.
     static const cmsCIEXYZ D50XYZ = {cmsD50X, cmsD50Y, cmsD50Z};
 
     return &D50XYZ;
 }
 
 const cmsCIExyY* CMSEXPORT cmsD50_xyY(void)
 {
+    // EmbedPDF: thread-confined runtime.
+    // This is equivalent to cmsXYZ2xyY(cmsD50_XYZ()), but avoids writing to a
+    // shared static on every call. ThreadSanitizer flags the old lazy
+    // recomputation when multiple documents create ICC transforms in parallel.
     static const cmsCIExyY D50xyY = {
         cmsD50X / (cmsD50X + cmsD50Y + cmsD50Z),
         cmsD50Y / (cmsD50X + cmsD50Y + cmsD50Z),
@@ -351,4 +356,3 @@ cmsBool CMSEXPORT cmsAdaptToIlluminant(cmsCIEXYZ* Result,
 
     return TRUE;
 }
-