mesa: enable NV_texture_barrier in GLES2+ (again)

mesa: enable NV_texture_barrier in GLES2+ (again)

This re-enables NV_texture_barrier in GL ES2+ contexts. This had
previously been tried, but caused CI issues and thus had been
reverted.

c7da969f8fe1 ("mesa: Enable NV_texture_barrier in GLES2+") was
buggy, as it added the es2 annotation to the category instead of
the function in the XML, which lead to the extension being
advertised, but calling glTextureBarrierNV in a GLES context
only yielded a GL_INVALID_OPERATION instead of the desired
barrier operation.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25728>