swrast: Use _mesa_format_convert to implement draw_rgba_pixels.
swrast: Use _mesa_format_convert to implement draw_rgba_pixels.

This is the only place that uses _mesa_unpack_color_span_float so after
this we should be able to remove that function.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler