This allows compositors which prefer to draw the decorations around
clients to do so, rather than letting GLFW draw its own decorations.
The appearance is thus entirely subject to the compositor used, but
should generally be better than the current solid colour decorations we
have, which we continue to use when the compositor doesn’t support this
protocol or tells us to draw the decorations ourselves.

This new protocol has been tested against wlroots’s rootston compositor.

Fixes #1257.