Bug 1721612 - Fix AdjustAdvancesForSyntheticBold to reliably handle n…

…egative adjustments. r=gfx-reviewers,lsalzman

This method (as its name suggests) was originally created to handle synthetic-bold,
which only ever increases the advance, and so its use of an unsigned value for the
adjustment worked ok.

But when applying tracking, the adjustment may be negative, and assigning this to
a uint32_t value takes us into undefined-behavior territory. It seems this worked
"as expected" on x86_64 (using modulo arithmetic), but on arm64 the value just
clamped to zero, and the intended negative tracking doesn't get applied.

Making it an int32_t results in consistent behavior across both architectures.

Differential Revision: https://phabricator.services.mozilla.com/D193288

gfx/thebes/gfxFont.cpp

@@ -843,7 +843,7 @@ bool gfxShapedText::FilterIfIgnorable(uint32_t aIndex, uint32_t aCh) {
 void gfxShapedText::AdjustAdvancesForSyntheticBold(float aSynBoldOffset,
                                                    uint32_t aOffset,
                                                    uint32_t aLength) {
-  uint32_t synAppUnitOffset = aSynBoldOffset * mAppUnitsPerDevUnit;
+  int32_t synAppUnitOffset = aSynBoldOffset * mAppUnitsPerDevUnit;
   CompressedGlyph* charGlyphs = GetCharacterGlyphs();
   for (uint32_t i = aOffset; i < aOffset + aLength; ++i) {
     CompressedGlyph* glyphData = charGlyphs + i;