Windows QPA: Fix over-large transparent tooltips

Setting transparency (WS_EX_LAYERED) causes a WM_PAINT to be sent to
the invisible windows, which causes a resize to the default size
(640x480) to be sent from QGuiApplicationPrivate::processExposeEvent().
Suppress these messages.

Fixes: QTBUG-75455
Change-Id: Idc540aa7f9bf0047e78ec7c27db260940483f7c4
Reviewed-by: Andre de la Rocha <andre.rocha@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1875,6 +1875,9 @@ bool QWindowsWindow::handleWmPaint(HWND hwnd, UINT message,
 {
     if (message == WM_ERASEBKGND) // Backing store - ignored.
         return true;
+    // QTBUG-75455: Suppress WM_PAINT sent to invisible windows when setting WS_EX_LAYERED
+    if (!window()->isVisible() && (GetWindowLong(hwnd, GWL_EXSTYLE) & WS_EX_LAYERED) != 0)
+        return false;
     // Ignore invalid update bounding rectangles
     RECT updateRect;
     if (!GetUpdateRect(m_data.hwnd, &updateRect, FALSE))