There were some magic conditions that decided when
meta_window_constrain() was to be called or not. Reasoning about and
changing these conditions were complicated, and in practice the caller
knows when constraining should be done. Lets change things by adding a
'constrain' flag to the move-resize flags that makes this clearer. This
way we can, if needed, have better control of when a window is
constrained or not without leaking that logic into the generic
to-constrain-or-not expression.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2338>
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