Use GL_OES_packed_depth_stencil on GLES2

If the GL_OES_packed_depth_stencil extension is available then we can
try creating a combined depth-stencil buffer with the
GL_DEPTH24_STENCIL8 format. This adds a private flag for the feature.

https://bugzilla.gnome.org/show_bug.cgi?id=666184



Reviewed-by: Robert Bragg <robert@linux.intel.com>