If you tried removing `culling: none` from any post-process materials,
everything would be fine in GL but Vulkan would be black because
our full-screen triangle was back-facing.
Continue reading only if you thrive in absurd situations.
- Metal and OpenGL define clip space as Y-up whereas Vulkan is Y-down.
- Metal and Vulkan define tex coords as Y-down whereas OpenGL is Y-up.