wayland: Only load HiddenPreviews options in X11

HiddenPreviews was a hack to not inform X11 that windows were minimised,
so that their previews kept working.

On wayland the user facing setting is not visible but the value is
loaded which is a bad combination.

CCBUG: 415286

src/options.cpp

@@ -787,17 +787,19 @@ void Options::loadConfig()
     }
     setGlPreferBufferSwap(c);
 
-    HiddenPreviews previews = Options::defaultHiddenPreviews();
-    // 4 - off, 5 - shown, 6 - always, other are old values
-    int hps = config.readEntry("HiddenPreviews", 5);
-    if (hps == 4) {
-        previews = HiddenPreviewsNever;
-    } else if (hps == 5) {
-        previews = HiddenPreviewsShown;
-    } else if (hps == 6) {
-        previews = HiddenPreviewsAlways;
-    }
-    setHiddenPreviews(previews);
+    if (kwinApp()->operationMode() == Application::OperationModeX11) {
+        HiddenPreviews previews = Options::defaultHiddenPreviews();
+        // 4 - off, 5 - shown, 6 - always, other are old values
+        int hps = config.readEntry("HiddenPreviews", 5);
+        if (hps == 4) {
+            previews = HiddenPreviewsNever;
+        } else if (hps == 5) {
+            previews = HiddenPreviewsShown;
+        } else if (hps == 6) {
+            previews = HiddenPreviewsAlways;
+        }
+        setHiddenPreviews(previews);
+    }
 
     auto interfaceToKey = [](OpenGLPlatformInterface interface) {
         switch (interface) {