gamescope: the micro-compositor 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.

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 support 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.

Keyboard shortcuts

Super + F : Toggle fullscreen Super + N : Toggle integer scaling

Examples

On any X11 or Wayland desktop running Mesa with this PR applied, you can set the Steam launch arguments of your game as follows:

// Upscale a 720p game to 1080p with integer scaling
gamescope -w 1280 -h 720 -W 1920 -H 1080 -n -- %command%

// Limit a vsynced game to 30 FPS (currently broken):
gamescope -r 30 -- %command%

// Run the game at 1080p, but scale output to a fullscreen 3440×1440 pillarboxed ultrawide window
gamescope -w 1920 -h 1080 -W 3440 -H 1440 -b -- %command%