Store whether a format is actually color- or depth-renderable.
Fix DownloadColorBufferImpl and suppress some unused variable warnings
Use GetBufferData instead of MapBuffer
Make some member functions const
Restore OpenGL state
Use aligned_allocator in HostBufferRT.

Alignment is currently fixed to 16-byte.
Start using render targets
Use render targets in vvRayRend
Use render targets in vvTexRend
Use render targets in vvSoftRayRend
Enable render targets in vox-desk-qt