Draft: Disable subpixel positioning when hinted

Fully hinted doesn't really mesh well with subpixel positioning as it doesn't handle the glyph being moved around in non-integer steps. So do what others do and disable subpixel positioning for anything but the slightest hinting.

This is an attempted fix at #511. However I'm not sure it's the right approach. This just makes all 16 subpixel variants be identical, but I think the proper fix is rounding coordinates somewhere earlier. I thought CAIRO_ROUND_GLYPH_POS_x was the key, but it seems to always be on?

Where is the glyph advancing handled? And is hint metrics also something I should deal with?