mesa/main: Modify generate_mipmap_compressed to cope with float textures

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.

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