Implement GPU-side corona fading

Implement GPU-side corona fading

Uses GL_ARB_query_buffer_object to retrieve query results directly into a
buffer. This allows corona fading to be moved into the fragment shader,
preventing a round-trip to the CPU and synchronous rendering. Depending on the
machine's configuration, it can give a dramatic performance boost.

From: Alex Goins <agoins@nvidia.com>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12094 d7cf8633-e32d-0410-b094-e92efae38249