As a hack for now, don't save/restore D32F_S8 texture data

* On nvidia it seems that doing glCopyImageSubData() on a D32F_S8 texture
  can cause serious problems, so ignore it for now. We can generally get
  away with it, as usually the only depth buffer in this format is the
  'main' depth buffer, which isn't used frame-to-frame.
Ditch explicit linear or point sampling in gl tex display shader

* It's handled already on the C++ side by binding a point or linear sampler
  to that slot, but also it seems that it's not well defined (that I can
  see) if texelFetch should sRGB decode or not. On nvidia it seems to, but
  on AMD it seems like it doesn't do the decode.