glx/dri3: add GPU offloading support.

glx/dri3: add GPU offloading support.

The differences with DRI2 GPU offloading are:
a) There's no logic for GPU offloading needed in the Xserver

b) for DRI2, the card would render to a back buffer, and
the content would be copied to the front buffer (the same buffers
everytime). Here we can potentially use several back buffers and copy
to buffers with no tiling to share with X. We send them with the
Present extension.

That means than the DRI2 solution is forced to have tearings with GPU
offloading. In the ideal scenario, this DRI3 solution doesn't have this
problem.

However without dma-buf fences, a race can appear (if the card is slow
and the rendering hasn't finished before the server card reads the buffer),
and then old content is displayed. If a user hits this, he should probably
revert to the DRI2 solution (LIBGL_DRI3_DISABLE). Users with cards fast
enough seem to not hit this in practice (I have an Amd hd 7730m, and I
don't hit this, except if I force a low dpm mode)

c) for non-fullscreen apps, the DRI2 GPU offloading solution requires
compositing. This DRI3 solution doesn't have this requirement. Rendering
to a pixmap also works.

d) There is no need to have a DDX loaded for the secondary card.

V4: Fixes some piglit tests

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Signed-off-by: Dave Airlie <airlied@redhat.com>