Implement GLX_EXT_buffer_age for DRI2

[Chris Wilson]

In order to suport GLX_EXT_buffer_age in DRI2, we need to pass back the
last swap buffer count that the back buffer was defined for. For
simplicity, we can reuse an existing field in the DRI2GetBuffers reply
that is not used by current drivers, the flags. Since we change the
interpretation of this flag, we also declare the semantic change with a
DRI2 parameter and depend upon the DDX to enable the change
responsibility (which is just a matter of reviewing whether the flags
field has ever been used for a non-zero value).

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=701801

This series add the core support to X, mesa and -ati/-nouveau.

[dri2proto] Declare DRI2ParamXHasBufferAge
[xorg 1/3] dri2: Allow GetBuffers to match any format
[xorg 2/3] dri2: Pass swap-interval=0 ScheduleSwap requests to the
[xorg 3/3] dri2: Reuse unused flags in GetBuffers protocol to pass
[xf86-video-ati] dri2: Enable BufferAge support
[xf86-video-nouveau] dri2: Enable BufferAge support
[mesa 7/9] glx/dri2: Add DRI2GetParam()
[mesa 8/9] glx/dri2: Move the wait after SwapBuffers into the next
[mesa 9/9] glx/dri2: Implement getBufferAge