mesa: Support GL_TEXTURE_BUFFER in GetTexLevelParameter[if]v in GL 3.1+.

mesa: Support GL_TEXTURE_BUFFER in GetTexLevelParameter[if]v in GL 3.1+.

The OpenGL 3.1 specification explicitly allows this.  Oddly, the
ARB_texture_buffer_object spec's issues section claims this isn't
allowed, but proceeds to explain that the extension simply doesn't edit
the underlying spec to allow it, and thus it didn't appear in the list
of legal texture targets.

Thus, this patch legalizes it only in 3.1+ contexts, but still returns
INVALID_ENUM in earlier contexts that expose ARB_texture_buffer_object.

Unfortunately, the behavior of the call is horrendously undefined.

Fixes oglconform's tbo/negative.textureParams test.

v2: Require desktop OpenGL.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>