glamor versus AIGLX GL context series

[Eric Anholt]

Here it is, at long last.  I was stumped for several days with a weird
fbo incomplete that was happening after my first round of fixes, and
it took a day's detour of cooking up a Xephyr with DRI3 to get a fast
debug environment and figure out what was going on (xephyr-glamor-egl
is that branch.  I warn you: that branch is almost entirely made up of
layering violations and bad assumptions, and should not be viewed by
people with a weak stomach).

With this series, I've run piglit on my glamor-using desktop, and also
played with some LIBGL_ALWAYS_INDIRECT=1 glxgears.  Those both used to
take down my server in different ways, so I think this fixes what we
need for 1.16.  However, swrast AIGLX (as used in xephyr-glamor-dri3)
has a problem with recursive flushes during the loader's GetImage, and
I don't have a solution to it.

This branch can be found on glamor-gl-context-2 of my tree.