st/mesa: Accelerate PBO uploads

st/mesa: Accelerate PBO uploads

Create a PIPE_BUFFER sampler view on the pixel-unpack buffer, and draw
the image on the texture with a fragment shader that maps fragment
coordinates to buffer coordinates.

Modifications by Nicolai Hähnle:
- various cleanups and fixes (e.g. error handling, corner cases)
- split try_pbo_upload into two functions, which will allow code to be
  shared with compressed texture uploads
- modify the source format selection to only test for support against
  the PIPE_BUFFER target

v2:
- update handling of TGSI_SEMANTIC_POSITION for recent changes in master
- MaxTextureBufferSize is number of texels, not bytes (Ilia Mirkin)
- only enable when integers are supported (Marek Olšák)
- try harder to hit the TextureBufferOffsetAlignment
- remove unnecessary MOV from the fragment shader

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>