Add pixman_glyph_cache_t API

Add pixman_glyph_cache_t API

This new API allows entire glyph strings to be composited in one go
which reduces overhead compared to multiple calls to
pixman_image_composite32().

The pixman_glyph_cache_t is a hash table that maps two keys (a "font"
and a "glyph" key, but they are just keys; there is no distinction
between them as far as pixman is concerned) to a glyph. Glyphs in the
cache can be composited through two new entry points
pixman_glyph_cache_composite_glyphs() and
pixman_glyph_cache_composite_glyphs_no_mask().

A glyph cache may only be inserted into when it is "frozen", which is
achieved by calling pixman_glyph_cache_freeze(). When
pixman_glyph_cache_thaw() is later called, if the cache has become too
crowded, some glyphs (currently the least-recently-used) will
automatically be evicted. This means that a user must ensure that all
the required glyphs are present in the cache before compositing a
string. The intended way to use the cache is like this:

        pixman_glyph_t glyphs[MAX_GLYPHS];

        pixman_glyph_cache_freeze (cache);

        for (i = 0; i < n_glyphs; ++i)
        {
            const void *g;

            if (!(g = pixman_glyph_cache_lookup (cache, font_key, glyph_key)))
            {
                img = <rasterize glyph as a pixman_image_t>;

                g = pixman_glyph_cache_insert (cache, font_key, glyph_key,
                                               glyph_origin_x, glyph_origin_y,
                                               img);

                if (!g)
                {
                    /* Clean up out-of-memory condition */
                    goto oom;
                }

                glyphs[i].pos_x = glyph_x_pos;
                glyphs[i].pos_y = glyph_y_pos;
                glyphs[i].glyph = g;
            }
        }

        pixman_composite_glyphs (op, src, dest, ..., cache, n_glyphs, glyphs);

        pixman_glyph_cache_thaw (cache);

V2:
- Move glyphs to front of the MRU list when they are used. Pointed
  out by Behdad Esfahbod.
- Composite glyphs with (white IN glyph) ADD mask in order to support
  mixed a8 and a8r8g8b8 glyphs. Also pointed out by Behdad.
- Add pixman_glyph_get_mask_format