glx/dri3: Don't fail on glXSwapBuffersMscOML(dpy, window, 0, 0, 0) (v2)

glx/dri3: Don't fail on glXSwapBuffersMscOML(dpy, window, 0, 0, 0) (v2)

glXSwapBuffersMscOML() with target_msc=divisor=remainder=0 gets
translated into target_msc=divisor=0 but remainder=1 by the mesa
api. This is done for server DRI2 where there needs to be a way
to tell the server-side DRI2ScheduleSwap implementation if a call
to glXSwapBuffers() or glXSwapBuffersMscOML(dpy,window,0,0,0) was
done. remainder = 1 was (ab)used as a flag to tell the server to
select proper semantic. The DRI3/Present backend ignored this
signalling, treated any target_msc=0 as glXSwapBuffers() request,
and called xcb_present_pixmap with invalid divisor=0, remainder=1
combo. The present extension responded kindly to this with a
BadValue error and dropped the request, but mesa's DRI3/Present
backend doesn't check for error codes. From there on stuff went
downhill quickly for the calling OpenGL client...

This patch fixes the problem.

v2: Change comments to be more clear, with reference to
relevant spec, as suggested by Eric Anholt.

Cc: "10.3 10.4" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Eric Anholt <eric@anholt.net>