[Mesa-dev] Add support GL_ARB_texture_compression_bptc in llvmpipe and softpipe.

[Denis Pauk]

Add code for reuse bptc decode logic from mesa/main/texcompress_bptc.c by make 
several function public(nonstatic) and define functions in texcompress_bptc_share.h. 

I have made minimal changes in code without possible performance improvements. And code 
decodes image by pixels instead possible decode full 4x4 block. Compress code works 
by compess full image at once (reuse compress function from texcompress_bptc). 

Checked on x86_64 by: 
* LIBGL_ALWAYS_SOFTWARE=true GALLIUM_DRIVER={llvmpipe,softpipe}
* piglit/bin/bptc-float-modes
* piglit/bin/bptc-modes
* piglit/bin/compressedteximage GL_COMPRESSED_RGBA_BPTC_UNORM
* piglit/bin/compressedteximage GL_COMPRESSED_SRGB_ALPHA_BPTC_UNORM
* piglit/bin/compressedteximage GL_COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT
* piglit/bin/compressedteximage GL_COMPRESSED_RGB_BPTC_SIGNED_FLOAT

Could you please review?

Best regards,
             Denis.