gallium: new raw gallium interface to support standalone tests

Provides basic window system integration behind a simple interface,
allowing tests to be written without dependency on either the driver
or window system.

With a lot of work, could turn into something like glut for gallium.
diff --git a/SConstruct b/SConstruct
index 2549a13..5d44b6e 100644
--- a/SConstruct
+++ b/SConstruct
@@ -52,7 +52,7 @@
 opts.Add(ListVariable('drivers', 'pipe drivers to build', default_drivers,
                      ['softpipe', 'failover', 'svga', 'i915', 'i965', 'trace', 'r300', 'identity', 'llvmpipe']))
 opts.Add(ListVariable('winsys', 'winsys drivers to build', default_winsys,
-                     ['xlib', 'vmware', 'i915', 'i965', 'gdi', 'radeon']))
+                     ['xlib', 'vmware', 'i915', 'i965', 'gdi', 'radeon', 'graw-xlib']))
 
 opts.Add(EnumVariable('MSVS_VERSION', 'MS Visual C++ version', None, allowed_values=('7.1', '8.0', '9.0')))