glx/dri3: Fix glXWaitForSbcOML() to handle targetSBC==0 correctly. (v2)

glx/dri3: Fix glXWaitForSbcOML() to handle targetSBC==0 correctly. (v2)

targetSBC == 0 is a special case, which asks the function
to block until all pending OpenGL bufferswap requests have
completed.

Currently the function just falls through for targetSBC == 0,
returning bogus results.

This breaks applications originally written and tested against
DRI2 which also rely on this not regressing under DRI3/Present,
e.g., Neuro-Science software like Psychtoolbox-3.

This patch fixes the problem.

v2: Simplify as suggested by Axel Davy. Add comments proposed
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>