gsk: Add font options to the glyph cache

gsk: Add font options to the glyph cache

The cairo_t that we create to render glyphs for
the glyph cache needs to match the font options
that are supposedly governing how glyphs are
drawn.

Since we allow font options to be different per
widget in gtk, we need to have them at least at
the level of individual render nodes. Adding them
to the lookup key for the glyph cache has the
side effect of solving another problem: We are
not flushing the cache when font options change.