[layout] Fix comparison of GlyphID and hb_codepoint_t

Before, the IntType::cmp functions providing this and was truncating
the hb_codepoint_t to 16bits before comparison.  I have no idea how
this was never discovered, and I'm too lazy to try to reproduce this
with Pango (which uses non-16bit codepoint numbers for missing glyphs).
1 file changed