mesa: Add texstore functions for BPTC-compressed textures

This adds compressors for all four of the BPTC compressed-texture formats. The
compressor is written from scratch and takes a very simple approach. It always
uses a single mode of the BPTC format (4 for unorm and 3 for half-floats) 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.

It's probably not really sensible to try to use BPTC compression at runtime
because for example with the Nvidia offline 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: Always use the custom compressor, even for the unorm formats. Fix the
    quantization step for the half-float format compressor. Fixed a typo which
    was breaking the right-hand edge of half-float textures with a width that
    isn't a multiple of four.
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