DRI2 direct rendering

[Kristian Høgsberg]

Hi,

I just committed the last big chunk of DRI2 work, the direct rendering
support.  With this, we can now do direct rendering to redirected
windows and GLX_EXT_texture_from_pixmap even works, so compiz (and
other Open GL compositing managers) can run in direct rendering mode
too.

I ended up rolling a couple of changes into the direct rendering work
and the commits in mesa and xserver grew much bigger than what I would
have liked.  Short story is that the DRI2 interface and the legacy
interface diverged and I had to break the DRI interface again.  So I
decided to make a couple of changes I'd been considering, most
noticably, the GLX specifc, open coded __GLcontextModes struct is no
longer part of the DRI driver interface, and with it glcore.h is also
gone.  This change is the cause of most of the code churn.  I could
have tried to split the commits up in a couple of independent commits,
but it would be a lot of work and I figured it'd be nice to only have
one commit that breaks the interface.

The upshot is that git xserver AIGLX needs git mesa DRI driver and for
DRI2, get the latest version of the xf86-video-intel intel-batchbuffer
branch too.

Kristian