output-swapchain-manager: new helper

This new helper assists compositors in allocating buffers for modesets. It degrades to different allocation parameters as needed, and should help with screens not turning on when multiple outputs are connected on some hardware (e.g. Intel).

For simplicity, the old logic to try allocating with explicit modifiers first and then fallback to implicit modifiers later is left as-is. We'll probably want to have more complicated logic instead in the future: try the fallback on one output at a time, and try dropping modifiers one by one instead of using implicit modifiers (at the cost of some combinatorial explosion).

Closes: #1873 (closed)
Depends on: !4548 (merged)
Cage patch: https://github.com/cage-kiosk/cage/pull/316