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This is still not perfect, because G2D can't saturate memory bandwidth
for this color depth (it is fillrate limited). We should emulate 16bpp blits
with 32bpp blits whenever it is possible.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Now source and destination pixmaps don't need to be the same for
using G2D acceleration (as long as both of them are allocated in
the framebuffer). This allows using G2D to copy pixels from DRI2
buffers to the framebuffer on the fallback path (when the window
of an OpenGL ES application is partially overlapped by some other
windows). Though it only works when composite extension is
disabled, for example by adding the following to xorg.conf:

    Section "Extensions"
        Option "Composite" "Disable"
    EndSection

If composite extension is enabled, windows have backing pixmaps, and
we have a longer chain of copies:

   DRI2 buffer -> backing pixmap -> framebuffer

Because backing pixmap is not allocated in a physically contiguous
memory, it can't be copied using G2D yet.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Wrap CreateGC function to add a hook for CopyArea operation, which
can be accelerated using G2D for the buffers inside of the visible
part of the framebuffer. In the future we may try to also ensure
that DRI2 buffers are copied using G2D instead of CPU in the case
if we hit the fallback path and can't avoid this copy.

Benchmark using "x11perf -scroll500 -copywinwin500":

=== ShadowFB (software rendering) ===

   3000 reps @   2.0308 msec (   492.0/sec): Scroll 500x500 pixels
   3000 reps @   1.9741 msec (   507.0/sec): Scroll 500x500 pixels
   3000 reps @   1.9826 msec (   504.0/sec): Scroll 500x500 pixels
   3000 reps @   1.9830 msec (   504.0/sec): Scroll 500x500 pixels
   3000 reps @   1.9965 msec (   501.0/sec): Scroll 500x500 pixels
  15000 trep @   1.9934 msec (   502.0/sec): Scroll 500x500 pixels

   1600 reps @   3.3054 msec (   303.0/sec): Copy 500x500 from window to window
   1600 reps @   3.3179 msec (   301.0/sec): Copy 500x500 from window to window
   1600 reps @   3.2263 msec (   310.0/sec): Copy 500x500 from window to window
   1600 reps @   3.2491 msec (   308.0/sec): Copy 500x500 from window to window
   1600 reps @   3.2357 msec (   309.0/sec): Copy 500x500 from window to window
   8000 trep @   3.2669 msec (   306.0/sec): Copy 500x500 from window to window

=== G2D (hardware acceleration) ===

   3000 reps @   2.1949 msec (   456.0/sec): Scroll 500x500 pixels
   3000 reps @   2.1929 msec (   456.0/sec): Scroll 500x500 pixels
   3000 reps @   2.1923 msec (   456.0/sec): Scroll 500x500 pixels
   3000 reps @   2.1889 msec (   457.0/sec): Scroll 500x500 pixels
   3000 reps @   2.1941 msec (   456.0/sec): Scroll 500x500 pixels
  15000 trep @   2.1926 msec (   456.0/sec): Scroll 500x500 pixels

   2800 reps @   1.8114 msec (   552.0/sec): Copy 500x500 from window to window
   2800 reps @   1.8103 msec (   552.0/sec): Copy 500x500 from window to window
   2800 reps @   1.8160 msec (   551.0/sec): Copy 500x500 from window to window
   2800 reps @   1.8099 msec (   553.0/sec): Copy 500x500 from window to window
   2800 reps @   1.8126 msec (   552.0/sec): Copy 500x500 from window to window
  14000 trep @   1.8120 msec (   552.0/sec): Copy 500x500 from window to window

CPU usage remains low when running this test with G2D acceleration enabled.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

This initial G2D support code can speed up moving windows in XFCE. Currently
disabled by default, but can be enabled by editing /etc/X11/xorg.conf and
adding the following line to the "Device" section:

        Option          "AccelMethod" "G2D"
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>