Avoid overflow in COLR bounds checks.

The values read into `base_glyphs_offset_v1` and `layer_offset_v1` may
be in the range 0xFFFFFFFD-0xFFFFFFFF. On systems where `unsigned long`
is 32 bits adding 4 to such values will wrap and pass bounds checks but
accessing values at such offsets will be out of bounds.

On the other hand `table_size` has already been tested to be at least
`COLRV1_HEADER_SIZE` (34) so it is safe to subtract 4 from it.

* src/sfnt/ttcolr.c (tt_face_load_colr): subtract 4 from `table_size`
instead of adding 4 to font data offsets in bounds checks

Fixes: https://crbug.com/1469348

src/sfnt/ttcolr.c

@@ -229,7 +229,7 @@
 
       base_glyphs_offset_v1 = FT_NEXT_ULONG( p );
 
-      if ( base_glyphs_offset_v1 + 4 >= table_size )
+      if ( base_glyphs_offset_v1 >= table_size - 4 )
         goto InvalidTable;
 
       p1                 = (FT_Byte*)( table + base_glyphs_offset_v1 );
@@ -249,7 +249,7 @@
 
       if ( layer_offset_v1 )
       {
-        if ( layer_offset_v1 + 4 >= table_size )
+        if ( layer_offset_v1 >= table_size - 4 )
           goto InvalidTable;
 
         p1            = (FT_Byte*)( table + layer_offset_v1 );