Add override option for compositor detection on Linux

[imsuck]

Currently Firefox will detect if a compositor is running. If so, Firefox would draw Client-Side Decorations for its right click menu, bookmark folders, etc. However, if the compositor stops running (sometimes I have to stop it manually), the CSD would not be drawn correctly.

Plus I prefer it when Firefox don't draw CSD and let the compositor do its job (ie. initially run Firefox without compositor running, open menus, then start compositor).

That's why I would like for Firefox to have an override option for compositor detection with values: off, on, auto or something similar to it.

[Jon]

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