glx/dri3: Implement LIBGL_SHOW_FPS=1 for DRI3/Present.

glx/dri3: Implement LIBGL_SHOW_FPS=1 for DRI3/Present.

v2: Use the UST value provided in the PRESENT_COMPLETE_NOTIFY event
    rather than gettimeofday(), which gives us the presentation time
    instead of the time when SwapBuffers was called.  Suggested by
    Keith Packard.  This relies on the fact that the X DRI3/Present
    implementations use microseconds for UST.

v3: Properly ignore PresentCompleteKindMSCNotify; multiply in 64 bits
    (caught by Keith Packard).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Keith Packard <keithp@keithp.com> [v3]
Reviewed-by: Marek Olšák <marek.olsak@amd.com> [v1]