This has a separate enable flag because this extension also requires
OES_geometry_shader. It is possible that some drivers may support
OpenGL ES 3.1 and ARB_texture_cube_map but not support
OES_geometry_shader.
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com> Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu> Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>