llvmpipe: don't use 3-component formats, except 32-bit x 3 formats
llvmpipe: don't use 3-component formats, except 32-bit x 3 formats

This basically disallows all 8-bit x 3 and 16-bit x 3 formats for
textures and render targets.  Some 3-component formats were already
disallowed before.  This avoids problems with GL_ARB_copy_image.

v2: the previous version of this patch disallowed all 3-component formats

Reviewed-by: Charmaine Lee <67ce69700661ea856134ddb10419a330e5f037d7@vmware.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler