mesa: Add texel fetch functions for BPTC-compressed textures

Adds functions to fetch from any of the four BPTC-compressed formats.

v2: Set the alpha component to 1.0 when fetching from the half-float formats
    instead of leaving it uninitialised.
mesa: Add texstore functions for BPTC-compressed textures

This adds compressors for all four of the BPTC compressed-texture formats. For
the RGB and SRGB normalized BPTC textures it works by first compressing each
4x4 block using the existing DXT3 compressor and then converting it to a BPTC
block. The BPTC block loses one bit of information on the green component of
the endpoints and one bit from the alpha values.

The compressor for the two half-float formats is written from scratch and
takes a very simple approach. It always uses mode 3 of the BPTC format and
picks the two endpoints by dividing the texels into those which have more or
less than the average luminance of the block and then calculating an average
color of the texels within each division.

The first approach using DXT3 gives much better results but neither is really
very good. However it's probably not really sensible to try to use BPTC
compression at runtime because for example with the Nvidia compression tool it
can take in the order of an hour to compress a full-screen image. With that in
mind I don't think it's worth having a proper compressor in Mesa and this
approach gives reasonable results for a usage that is basically a corner case.

v2: Fix the quantization step for the half-float format compressor.
mesa/main: Modify generate_mipmap_compressed to cope with float textures

Once we add BPTC texture support we will need to generate mipmaps for
compressed floating point textures too. Most of the code seems to already be
there but it just needs a few extra lines to get it to use GL_FLOAT instead of
GL_UNSIGNED_BYTE as the type for the temporary buffers.
i965: Enable the GL_ARB_texture_compression_bptc extension

Enables the BPTC extension on Gen>=7 and adds the necessary format mappings to
get the right surface type value.
swrast: Enable GL_ARB_texture_compression_bptc

Enables BPTC texture compression on the software rasterizer.
docs: Update release notes and GL3.txt for GL_ARB_texture_compression_bptc