msaa: Generate proper error for operations prohibited on MSAA buffers.

msaa: Generate proper error for operations prohibited on MSAA buffers.

From the GL 3.0 spec, section 4.3.3, in the documentation for
CopyPixels():

    "An INVALID_OPERATION error will be generated if the object bound
    to READ_FRAMEBUFFER_BINDING is framebuffer complete and the value
    of SAMPLE_BUFFERS is greater than zero."

The same applies to CopyTexImage...() and CopyTexSubImage...()
functions, since they are defined in terms of CopyPixels().

Previously we were generating an INVALID_FRAMEBUFFER_OPERATION error
in these cases.

Fixes piglit tests
"EXT_framebuffer_multisample/negative-{copypixels,copyteximage}".

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>