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GL only requires framebuffers whose depth and stencil attachments refer to the
same image; anything else may be answered GL_FRAMEBUFFER_UNSUPPORTED, and both
backends' real targets do exactly that - DirectVulkan cannot form two separate
attachments at all, and the ES drivers behind DirectGLES return UNSUPPORTED for
a separate depth renderbuffer plus stencil renderbuffer.
The frontend already knew how to detect the configuration, but only consulted it
for DirectVulkan. On DirectGLES it answered GL_FRAMEBUFFER_COMPLETE for a
framebuffer the driver had rejected, so every clear and draw against it was
silently dropped and the results read back as zeros - which is what
KHR-GL3x.packed_depth_stencil.verify_mixed_attachments saw. (That test
explicitly tolerates GL_FRAMEBUFFER_UNSUPPORTED; what it cannot survive is being
told the framebuffer works.)
Turned into a backend capability rather than a backend-type check, probed once
at init from a scratch framebuffer the same way the format-capability cache is,
so a driver that does support the configuration keeps using it. Defaults to
supported, leaving any backend that does not set it on the permissive path.
Fixes KHR-GL3{2,3}.packed_depth_stencil.verify_mixed_attachments for both
formats; DirectVulkan re-verified unchanged at 23/25 pass + 2 not-supported.