wl-mirror - a simple Wayland output mirror client

wl-mirror attempts to provide a solution to sway's lack of output
mirroring by mirroring an output onto a client surface.

Features
* Mirror an output onto a resizable window
* Mirror an output onto another output by fullscreening the window
* Reacts to changes in output scale (including fractional scaling)
* Preserves aspect ratio
* Corrects for flipped or rotated outputs
* Supports custom flips or rotations
* Supports mirroring custom regions of outputs
* Supports receiving additional options on stdin for changing the
* mirrored screen or region on the fly (works best when used with
* pipectl)