Bug 1786525 - Don't update untransformed anchor rect when moved by move-to-rect. r=stransky

Otherwise, it changes the move-to-rect inputs, which can change the
output as well, making us move the anchor all the way to the right.

You could, I guess, consider this a mutter bug of sorts, because it
feels weird that you pass it an anchor that has been a `move-to-rect`
output and you get another rect as an output.

But also, it's kinda silly that we're doing that to begin with, so avoid
it by telling the popup frame whether it's been positioned / moved by
move-to-rect (and keeping the anchor in that case).

The reason this works on my setup without "Large text" is just dumb luck
(the front-end computes a max-height for the panel that is small enough
to fit on the screen).

Differential Revision: https://phabricator.services.mozilla.com/D155406