gamescope: the program formerly known as steamcompmgr

In an embedded session usecase, gamescope does the same thing as steamcompmgr, but with less extra copies and latency:

 - It's getting game frames through Wayland by way of Xwayland, so there's no copy within X itself before it gets the frame.
 
 - It can use DRM/KMS to directly flip game frames to the screen, even when stretching or when notifications are up, removing another copy.
 
 - When it does need to composite with the GPU, it does so with async Vulkan compute, meaning you get to see your frame quick even if the game already has the GPU busy with the next frame.
 
It also runs on top of a regular desktop, the 'nested' usecase steamcompmgr didn't support.

 - Because the game is running in its own personal Xwayland sandbox desktop, it can't interfere with your desktop and your desktop can't interfere with it.
 
 - You can spoof a virtual screen with a desired resolution and refresh rate as the only thing the game sees, and control/resize the output as needed. This can be useful in exotic display configurations like ultrawide or multi-monitor setups that involve rotation.
 
The features of that second part are working, but aren't exposed to the user yet.

It runs on Mesa+AMDGPU, and could be made to run on other Mesa/DRM drivers with minimal work. Can support NVIDIA if/when they supported accelerated Xwayland.

If running RadeonSI clients, currently have to set R600_DEBUG=nodcc, or corruption will be observed until the stack picks up DRM modifiers support.