glx: Pass NULL DRI drawables into the DRI driver for None GLX drawables

glx: Pass NULL DRI drawables into the DRI driver for None GLX drawables

GLX_ARB_create_context allows making a GLX context current with None
drawable and readables, but this was never implemented correctly in GLX.
We would create a __DRIdrawable for the None GLX drawable and pass that
to the DRI driver and that would somehow work.  Now it's somehow broken.

The way this should have worked is that we pass a NULL DRI drawable
to the DRI driver when the GLX user calls glXMakeContextCurrent()
with None for drawable and readables.

https://bugs.freedesktop.org/show_bug.cgi?id=74143
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>