Reset r_shadow_occlusion_buf on vid_restart. Fixes query buffer dangling pointer

Reset r_shadow_occlusion_buf on vid_restart. Fixes query buffer dangling pointer

Basically, when coronas appear when r_coronas_occlusionquery is 1, the engine
binds to a query buffer and stores a pointer to it in r_shadow_occlusion_buf.
Upon vid_restart, somewhere along the way, the buffer is cleared but the
original pointer is kept, resulting in a crash next time a corona appears.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12541 d7cf8633-e32d-0410-b094-e92efae38249