When the GLX or the EGL backend is destroyed, it is going to reset the
RenderLoop state, including the number of frames in flight. It does so
because of the historical reasons. At the time, there was no output frame
object to track the lifecycle of a frame.

After introducing the OutputFrame and hooking it into the RenderLoop,
the pending frame count will be reset automatically in RenderLoop when
the GLX or the EGL backend is destroyed. But we forgot to remove
the invalidate() function calls. So, when the GLX backend goes down, it
resets the pending frame count to zero and then it destroys the pending
OutputFrame object, which would result in decreasing the pending frame
count by 1 and triggering an assert in the RenderLoopPrivate::notifyFrameDropped()
function.

Since there is the OutputFrame helper now, the invalidate() function
can be dropped. Technically, the invalidate function did more than just
reset pendingFrameCount to 0, for example also stop the compositeTime.
But that should be fairly harmless new behavior.