We have a frame clock that ensures rendering is done as per the
output vsync. There is no need to have Mesa do the same for us.

This, most notably, ensures Mesa doesn't schedule frame callbacks
that will be left unattended if the compositor stops throttling
frames for its surface, this is eg. the case if the toplevel is
moved to another workspace.

Also, given a SwapInterval!=0 will always bring these unexpected
side effects, check that it's possible to disable it, and spew
a debug message if that isn't the case.

https://bugzilla.gnome.org/show_bug.cgi?id=769835