Detect clusters in Indic scripts where an initial RA is reordered after the following consonant and vowel. Bug 428317, r=roc

gfx/thebes/src/gfxAtsuiFonts.cpp

@@ -1227,16 +1227,26 @@ PostLayoutCallback(ATSULineRef aLine, gfxTextRun *aRun,
         while (glyphCount < numGlyphs) {
             ATSLayoutRecord *glyph = &glyphRecords[glyphIndex + direction*glyphCount];
             PRUint32 glyphOffset = glyph->originalOffset;
+            PRUint32 nextIndex = isRTL ? glyphIndex - 1 : glyphIndex + 1;
+            PRUint32 nextOffset;
+            if (nextIndex >= 0 && nextIndex < numGlyphs) {
+                ATSLayoutRecord *nextGlyph = &glyphRecords[nextIndex + direction*glyphCount];
+                nextOffset = nextGlyph->originalOffset;
+            }
+            else
+                nextOffset = glyphOffset;
             allFlags |= glyph->flags;
-            if (glyphOffset <= lastOffset) {
+            if (glyphOffset <= lastOffset || nextOffset <= lastOffset) {
                 // Always add the current glyph to the ligature group if it's for the same
                 // character as a character whose glyph is already in the group,
                 // or an earlier character. The latter can happen because ATSUI
-                // sometimes visually reorders glyphs; e.g. DEVANAGARI VOWEL I
-                // can have its glyph displayed before the glyph for the consonant that's
-                // it's logically after (even though this is all left-to-right text).
-                // In this case we need to make sure the glyph for the consonant
-                // is added to the group containing the vowel.
+                // sometimes visually reorders glyphs. One case of this is that DEVANAGARI
+                // VOWEL I can have its glyph displayed before the glyph for the consonant
+                // that it's logically after (even though this is all left-to-right text).
+                // Another case is that a sequence of RA; VIRAMA; <consonant> ; <vowel> is
+                // reordered to <consonant> ; <vowel> ; RA; VIRAMA.
+                // In these cases we need to make sure that the whole sequence of glyphs is
+                // processed as a single cluster.
             } else {
                 // We could be at the end of a character group
                 if (glyph->glyphID != ATSUI_SPECIAL_GLYPH_ID) {