mesa: Allow glReadBuffer(GL_NONE) for winsys framebuffers.

mesa: Allow glReadBuffer(GL_NONE) for winsys framebuffers.

Previously, Mesa code assumed that glReadBuffer(GL_NONE) was only
valid for user-created framebuffer objects.  However, the spec is
quite clear that is should also be valid for the default framebuffer.
From section 18.2.1 ("Obtaining Pixels from the Framebuffer") of the
GL 4.3 spec:

    "When READ_FRAMEBUFFER_BINDING is zero, i.e. the default
    framebuffer, src must be one of the values listed in table 17.4,
    including NONE."

Similar language exists in the GLES 3.0 spec, and in desktop GL all
the way back to ARB_framebuffer_object.

Partially fixes GLES3 conformance test "CoverageES30.test".

NOTE: This is a candidate for stable branches.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>