When glsurface is available, use GL to upscale the render buffer.
Additionally, fix build issues with sdlayer12 introduced by GLAD changes (r6656).
git-svn-id: https://svn.eduke32.com/eduke32@6939 1a8010ca-5511-0410-912e-c29ae57300e0
Fix root causes of issues causing crash and black screen in Mapster32 when switching back and forth between 3D & 2D modes
git-svn-id: https://svn.eduke32.com/eduke32@6931 1a8010ca-5511-0410-912e-c29ae57300e0