WebGL doesn't allow you to separately attach buffers to the
STENCIL_ATTACHMENT and DEPTH_ATTACHMENT framebuffer attachment points
and instead requires you to use the DEPTH_STENCIL_ATTACHMENT whenever
you want a depth and stencil buffer.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit ec7b6360c9c4e45e0b113f9dca7bb1502e7e93be)