webgl: use DEPTH_STENCIL_ATTACHMENT
webgl: use DEPTH_STENCIL_ATTACHMENT

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 <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

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