Speed up _pixman_composite_glyphs_no_mask()

Speed up _pixman_composite_glyphs_no_mask()

Bypass much of the overhead of pixman_image_composite32() by only
computing the composite region once instead of once per glyph, and by
only looking up the composite function whenever the glyph format or
flags change.

As part of this, the pixman_compute_composite_region32() was renamed
to _pixman_compute_composite_region32() and exported in
pixman-private.h.

I couldn't find a trace that would reliably demonstrate that this is
actually an improvement by itself (since _pixman_composite_glyphs_no_mask()
is called so rarely), but together with the following optimization for
solid sources, there is a small but reliable improvement to the
xfce4-a1-terminal cairo trace.