Replace the font argument with a hb_paint_context_t
that carries the font, the palette index and the
foreground color.
The hb_font_paint_glyph() api now takes the palette
index and the foreground color as extra arguments.
Update all callers and regenerate test results.
This will lead to easier implementations.
At the same time, we change the push_clip_glyph
callback to use the font as-is, no unscaling needed.
Update all callers and expected test results.