Support the new DRI2 protocol request, DRI2SwapBuffers, in both direct
and indirect rendering context.  This request allows the display server
to optimize back->front swaps (e.g. through page flipping) and allows us
to more easily support other GLX features like swap interval and the OML
sync extension in DRI2.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>