single-pixel-buffer-v1: new protocol implementation
This implements the single-pixel-buffer-v1 protocol 1, to allow clients to create buffers with a single color.
In the future, it would be nice to optimize this case by not creating a 1x1 texture in renderers. Needs a few adjustments to the wlroots APIs to make this possible.
Sway patch: https://github.com/swaywm/sway/pull/7053