[GTK] Don't notify cursor when cursorRect is default-initialized

https://bugs.webkit.org/show_bug.cgi?id=218148

Reviewed by Michael Catanzaro.

For input methods in preedit mode, editor state update functions could
get an IntRect that is default-initialized (with x, y, width and height
all 0), which makes the input method popup at the top left corner
instead of its expected position.

* Source/WebKit/UIProcess/API/glib/InputMethodFilter.cpp:
(WebKit::InputMethodFilter::notifyCursorRect): Modified to check if
cursorRect is default-initialized.

Canonical link: https://commits.webkit.org/278261@main

Source/WebKit/UIProcess/API/glib/InputMethodFilter.cpp

@@ -215,6 +215,10 @@ void InputMethodFilter::notifyCursorRect(const IntRect& cursorRect)
     if (!isEnabled() || !m_context)
         return;
 
+    // Don't notify cursor area when cursorRect is default-initialized (location and size are all 0).
+    if (!cursorRect.x() && !cursorRect.y() && !cursorRect.width() && !cursorRect.height())
+        return;
+
     // Don't move the window unless the cursor actually moves more than 10
     // pixels. This prevents us from making the window flash during minor
     // cursor adjustments.