Fixes https://bugs.freedesktop.org/show_bug.cgi?id=18710 .
As this can't work without new EXA_PREPARE_AUX* indices, this requires a major
version bump, so we can also drop the UploadToScratch driver hook and
ExaOffscreenSwap*(). So this also fixes
http://bugs.freedesktop.org/show_bug.cgi?id=20213 .
Moreover, introduce EXA_DRIVER_KNOWN_MAJOR to break compilation of drivers
which may not be able to handle EXA_PREPARE_AUX*, giving instructions how to
make them build again in the #error message.
Signed-off-by: Michel Dänzer <daenzer@vmware.com>