Composite: Enforce KWIN_COMPOSE

Don't fall back into other composition systems if KWIN_COMPOSE is
explicitly set. In such case, just exit().

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>

src/composite.cpp

@@ -289,6 +289,9 @@ bool Compositor::setupStart()
 
         if (stop) {
             break;
+        } else if (qEnvironmentVariableIsSet("KWIN_COMPOSE")) {
+            qCCritical(KWIN_CORE) << "Could not fulfill the requested compositing mode in KWIN_COMPOSE:" << type << ". Exiting.";
+            qApp->quit();
         }
     }