Adds explicit mention of the conditions that may cause implementations
to return FRAMEBUFFER_UNSUPPORTED for framebuffers with separate depth
and stencil. (The spec currently allows implementations to return
FRAMEBUFFER_UNSUPPORTED for any implementation dependent reason, so this
is not a spec change, but it is more helpful for users of the API if
these conditions are set out in the spec.)
Adds explicit mention of the conditions that may cause implementations to return FRAMEBUFFER_UNSUPPORTED for framebuffers with separate depth and stencil. (The spec currently allows implementations to return FRAMEBUFFER_UNSUPPORTED for any implementation dependent reason, so this is not a spec change, but it is more helpful for users of the API if these conditions are set out in the spec.)