mesa: Use _mesa_format_convert to implement texstore_rgba.
mesa: Use _mesa_format_convert to implement texstore_rgba.

Notice that _mesa_format_convert does not handle byte-swapping scenarios,
GL_COLOR_INDEX or MESA_FORMAT_YCBCR(_REV), so these must be handled
separately.

Also, remove all the code that goes unused after using _mesa_format_convert.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler