The very base class is the wrong place for it because
`FreeModSelectOverlay` inherits from it, and that one has different
assumptions and rules than "user" selection. In particular, in non-user
selection, more than one rate adjust mod may be active, which breaks the
mod speed hotkey's basic assumptions.
Being able to flip doesn't really feel all that good and `master` was
already clamping, so let's just bring that back for now. Flipping can be
reconsidered in a follow-up if it actually can be made to behave well.
Not only is this simpler, but it also is more correct (for explanation
why, try holding both shift keys while dragging, and just releasing one
of them - the previous code would briefly turn aspect ratio off).