Fix mip display
Proper SRGB handling in opengl. See gl-320-fbo-srgb

* We create fake backbuffer as SRGB if the original default backbuffer was
  SRGB in the program.
* All of our output backbuffers are also SRGB, and we enable
  GL_FRAMEBUFFER_SRGB when writing to them.
* Add the 'fake' srgb curve applied to linear data to display it as if it
  were srgb, which tends to look intuitively correct even if it's not 100%
  accurate. See the behaviour existing in D3D11 already.
Don't use mipmapping for point sampler

* This fixes issues where textures wouldn't render because they weren't
  mipmap complete, and although I was point sampling a single mip the point
  sampler was referencing the mipchain.