This is the initial implementation based on DRM dumb buffers,
which cleans up various things at the same time:
- DRM output is separate from GBM and DRM dumb buffer displays.
The latter two re-use the former.
- The timer refresh for the Vulkan KHR Display rendering lives
now with the KHR display code, in hope to be replaced with a
better mechanism in the future.
What's missing still is support for partial updates as well as
rendering with our own software renderer.
This might fix the issue with resizing causing swapchain re-creation
errors, because "Swapchain creation no longer returns an error when the
swapchain extent doesn't match the current surface extent."
cc #3850
