Only accept usable bounds if they differ from the full bounds

Some platforms (Wayland) just report the full display area, so we
incorrectly assume the entire display is usable for a window.

Fixes #2056

app/utils.h

@@ -12,6 +12,7 @@ namespace WMUtils {
     bool isRunningWayland();
     bool isRunningWindowManager();
     bool isRunningDesktopEnvironment();
+    bool hasDisplayUsableBounds();
     QString getDrmCardOverride();
     bool isGpuSlow();
 }