Refactored lightmap update handling and added cvars to replace gl_nopartialtextureupd...

Refactored lightmap update handling and added cvars to replace gl_nopartialtextureupdates cvar with finer-grained control:
* r_q1bsp_lightmap_updates_enabled (default 1 - shouldn't turn this off)
* r_q1bsp_lightmap_updates_combine (default 2 - upload full lightmap texture)
* r_q1bsp_lightmap_updates_hidden_surfaces (default 0 - if turned on it may improve fps when the view is moving/turning by only uploading lightmaps on certain frames)

Previously there was a bug that made hidden surfaces get lightmap updates, which was never intended, but a cvar was added to allow the behavior to be toggled in case it has some use.

Overall this change performs better on AMD drivers for Windows, and is expected to perform a lot better on all Mesa drivers for Linux as they have even worse performance with partial texture update locking, it may reduce performance on NVIDIA drivers for Windows as they have always had good handling of thousands of small partial texture updates.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@13056 d7cf8633-e32d-0410-b094-e92efae38249