via EXA composite acceleration

[Thomas Hellström]

Hi!

I've updated bug #5180 with a first shot at render acceleration for via 
through EXA composite. While small composite operations in hardware are 
rather slow, at least it makes using a composite manager with shadows 
and translucency enjoyable. The code makes use of drm for uploads, 
downloads and AGP scratch areas, but should work well also without DRI 
enabled (or compiled in) with the exception of short pauses during big 
downloads.

<https://bugs.freedesktop.org/show_bug.cgi?id=5180>

While the patch is made against 6.9 I'd like to bring the code in on the 
7.0 tree if the protests aren't too loud.

/Thomas