mesa: Add texel fetch functions for BPTC-compressed textures

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. Don't linearize the alpha component
    when fetching from sRGB.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>