This new API allows glyphs to be cached in a data structure in pixman,
and entire glyph strings to be composited in one go.

Also bump pixman dependency to 0.27.2.

Results from the cairo peformance test suite running against Xvfb with
a screen size of 1680x1050@32bpp:

Speedups
========
 xlib          firefox-talos-gfx  12416.63 -> 3603.93   3.45x speedup
██▌
 xlib          xfce4-terminal-a1   1727.57 -> 1048.85:  1.65x speedup
▋
 xlib                  evolution   1370.49 -> 869.34:   1.58x speedup
▋
 xlib         gnome-terminal-vim   1832.83 -> 1251.94:  1.46x speedup
▌
 xlib                    poppler   1519.70 -> 1204.05:  1.26x speedup
▎
 xlib       firefox-planet-gnome   6982.55 -> 5598.16:  1.25x speedup
▎
 xlib                  ocitysmap   1142.77 -> 1071.53:  1.07x speedup
▏

No slowdowns were reported.

Results of x11perf -aa10text:

Before:

      8000000 reps @   0.0007 msec (1450000.0/sec)
      8000000 reps @   0.0007 msec (1460000.0/sec)
      8000000 reps @   0.0007 msec (1460000.0/sec)
      8000000 reps @   0.0007 msec (1470000.0/sec)
      8000000 reps @   0.0007 msec (1480000.0/sec)
     40000000 trep @   0.0007 msec (1460000.0/sec)

After:

     32000000 reps @   0.0002 msec (4910000.0/sec)
     32000000 reps @   0.0002 msec (4830000.0/sec)
     32000000 reps @   0.0002 msec (4890000.0/sec)
     32000000 reps @   0.0002 msec (4830000.0/sec)
     32000000 reps @   0.0002 msec (4900000.0/sec)
    160000000 trep @   0.0002 msec (4870000.0/sec)

Version 2: Destroy the glyph cache at server regen time

Acked-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Soren Sandmann <ssp@redhat.com>