UPSTREAM: mesa/get: allow NV_pixel_buffer_object constants in GLES2

The NV_pixel_buffer_object extension can be available in a GLES2
context, so the PIXEL_PACK_BUFFER_BINDING/PIXEL_UNPACK_BUFFER_BINDING
constants should also be available.

Tested on 8086:2e12, "Mesa DRI Intel(R) Q45/Q43 (ELK)".

(cherry picked from commit 37c3e16d3567928ad74b08c7408a8bf117b16532
    https://gitlab.freedesktop.org/mesa/mesa.git main)

BUG=b:184383677
TEST=Modify browser to use an ES2 context as described in linked bug, check that UI works on the HP Compaq 8000 Elite

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5655
Signed-off-by: Nicholas Bishop <nicholasbishop@google.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13862>
(cherry picked from commit 37c3e16d3567928ad74b08c7408a8bf117b16532)
Change-Id: Ia48601b308ebf5d59c1ab8386688e77d2d3bf770
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/mesa/+/3307340
Reviewed-by: Michael Pobega <pobega@google.com>
1 file changed