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.cpp

@@ -221,3 +221,12 @@ CInputPopup* CInputMethodRelay::popupFromCoords(const Vector2D& point) {
 
     return nullptr;
 }
+
+CInputPopup* CInputMethodRelay::popupFromSurface(const wlr_surface* surface) {
+    for (auto& p : m_vIMEPopups) {
+        if (p->getWlrSurface() == surface)
+            return p.get();
+    }
+
+    return nullptr;
+}