Disable translucency when KDDW_NO_TRANSLUCENCY env var is set

Useful for running on weird window managers which don't support
transparent windows.

Changelog

@@ -16,6 +16,8 @@
   for drop indicators while dragging.
   - Fixed embedding the main window into a non-kddw main window (#204, #168)
   - Fixed crash with an MFC application
+  - Allows to disable translucency via the KDDW_NO_TRANSLUCENCY env var, useful
+    in case running on weird Window Managers.
 
 * v1.3.2 (unreleased)
   -

src/private/Utils_p.h

@@ -131,6 +131,9 @@ inline void activateWindow(QWindow *window)
 
 inline bool windowManagerHasTranslucency()
 {
+    if (qEnvironmentVariableIsSet("KDDW_NO_TRANSLUCENCY"))
+        return false;
+
 #ifdef QT_X11EXTRAS_LIB
     if (qApp->platformName() == QLatin1String("xcb"))
         return QX11Info::isCompositingManagerRunning();