stb_truetype: GPOS parsing fixes

Glyphs not assigned a glyph class should default to class 0 as
per the OpenType spec. Also, change the code to treat malformed
data as an error to be handled, not an assert, and change the
way the version checking works.

Fixes the issue mentioned in PR #1035. Also, this part of the
code is indented incorrectly; will fix that in a subsequent
commit.

stb_truetype.h

@@ -2479,12 +2479,13 @@ static stbtt_int32  stbtt__GetGlyphClass(stbtt_uint8 *classDefTable, int glyph)
         } break;
 
         default: {
-            // There are no other cases.
-            STBTT_assert(0);
+            // Unsupported defition type; return an error.
+            return -1;
         } break;
     }
 
-    return -1;
+    // "All glyphs not assigned to a class fall into class 0". (OpenType spec)
+    return 0;
 }
 
 // Define to STBTT_assert(x) if you want to break on unimplemented formats.
@@ -2533,20 +2534,15 @@ static stbtt_int32  stbtt__GetGlyphGPOSInfoAdvance(const stbtt_fontinfo *info, i
                             int straw, needle;
                             stbtt_uint16 valueFormat1 = ttUSHORT(table + 4);
                             stbtt_uint16 valueFormat2 = ttUSHORT(table + 6);
+                            if (valueFormat1 == 4 && valueFormat2 == 0) { // Support more formats?
                                 stbtt_int32 valueRecordPairSizeInBytes = 2;
                                 stbtt_uint16 pairSetCount = ttUSHORT(table + 8);
                                 stbtt_uint16 pairPosOffset = ttUSHORT(table + 10 + 2 * coverageIndex);
                                 stbtt_uint8 *pairValueTable = table + pairPosOffset;
                                 stbtt_uint16 pairValueCount = ttUSHORT(pairValueTable);
                                 stbtt_uint8 *pairValueArray = pairValueTable + 2;
-                            // TODO: Support more formats.
-                            STBTT_GPOS_TODO_assert(valueFormat1 == 4);
-                            if (valueFormat1 != 4) return 0;
-                            STBTT_GPOS_TODO_assert(valueFormat2 == 0);
-                            if (valueFormat2 != 0) return 0;
 
-                            STBTT_assert(coverageIndex < pairSetCount);
-                            STBTT__NOTUSED(pairSetCount);
+                                if (coverageIndex >= pairSetCount) return 0;
 
                                 needle=glyph2;
                                 r=pairValueCount-1;
@@ -2569,12 +2565,14 @@ static stbtt_int32  stbtt__GetGlyphGPOSInfoAdvance(const stbtt_fontinfo *info, i
                                         return xAdvance;
                                     }
                                 }
+                            } else
+                                return 0;
                         } break;
 
                         case 2: {
                             stbtt_uint16 valueFormat1 = ttUSHORT(table + 4);
                             stbtt_uint16 valueFormat2 = ttUSHORT(table + 6);
-
+                            if (valueFormat1 == 4 && valueFormat2 == 0) { // Support more formats?
                                 stbtt_uint16 classDef1Offset = ttUSHORT(table + 8);
                                 stbtt_uint16 classDef2Offset = ttUSHORT(table + 10);
                                 int glyph1class = stbtt__GetGlyphClass(table + classDef1Offset, glyph1);
@@ -2582,26 +2580,21 @@ static stbtt_int32  stbtt__GetGlyphGPOSInfoAdvance(const stbtt_fontinfo *info, i
 
                                 stbtt_uint16 class1Count = ttUSHORT(table + 12);
                                 stbtt_uint16 class2Count = ttUSHORT(table + 14);
-                            STBTT_assert(glyph1class < class1Count);
-                            STBTT_assert(glyph2class < class2Count);
 
-                            // TODO: Support more formats.
-                            STBTT_GPOS_TODO_assert(valueFormat1 == 4);
-                            if (valueFormat1 != 4) return 0;
-                            STBTT_GPOS_TODO_assert(valueFormat2 == 0);
-                            if (valueFormat2 != 0) return 0;
+                                if (glyph1class < 0 || glyph1class >= class1Count) return 0; // malformed
+                                if (glyph2class < 0 || glyph2class >= class2Count) return 0; // malformed
 
-                            if (glyph1class >= 0 && glyph1class < class1Count && glyph2class >= 0 && glyph2class < class2Count) {
                                 stbtt_uint8 *class1Records = table + 16;
                                 stbtt_uint8 *class2Records = class1Records + 2 * (glyph1class * class2Count);
                                 stbtt_int16 xAdvance = ttSHORT(class2Records + 2 * glyph2class);
                                 return xAdvance;
-                            }
+                            } else
+                                return 0;
                         } break;
 
                         default: {
-                            // There are no other cases.
-                            STBTT_assert(0);
+                            // Unsupported definition type
+                            return 0;
                             break;
                         };
                     }