IME: fix IME popup mouse inputs (again) (#5417)

`lastBoxLocal`'s size should be the actual popup's size instead of the cursor
rectangle's size. Also, the rectangle position is now relative to the popup.
(Actually fixes #5255 imho.)

One thing #3922 missed was handling focus held by buttons. Let's hope I get
it right this time.

src/managers/input/InputMethodRelay.hpp

@@ -35,6 +35,7 @@ class CInputMethodRelay {
     void                 removePopup(CInputPopup*);
 
     CInputPopup*         popupFromCoords(const Vector2D& point);
+    CInputPopup*         popupFromSurface(const wlr_surface* surface);
 
     void                 updateAllPopups();