intel: update intel_create_renderbuffer(format), add XRGB support

intel: update intel_create_renderbuffer(format), add XRGB support

Pass a gl_format to intel_create_renderbuffer() instead of GLenum.
Add cases for MESA_FORMAT_XRGB8888 textures and renderbuffers.
However, we don't yet create any renderbuffers or textures with that
format.  It seems the default alpha value is zero instead of one.
Need to investigate that first.