cmap format 4 subtable checking

[mikeday]

ots/src/cmap.cc

Line 242 in f87b455

if (glyph >= num_glyphs) {

I think it needs to add id_delta here to get the correct glyph index before checking the glyph is within range and non-negative.

[khaledhosny]

Sounds about right. Do you have fonts that fails without changing this? I’m rather surprised this didn’t surface before, if it is really an issue.

[mikeday]

I couldn't find a font that triggers an actual error, because the font that we hit just returns the wrong glyph if you don't add id_delta, but it's still a valid glyph, not out of range. But it's possible that there is a font out there with a broken id_delta value that this won't catch.

[khaledhosny]

OK, I’ll see if I can make some test font to add to the test suite.