Allow desktop settings to be changed on a single-screen system when a multi-screen configuration was previously set

This resolves Bug 1684

kcontrol/background/bgrender.cpp

@@ -1208,6 +1208,10 @@ void KVirtualBGRenderer::initRenderers()
     m_bCommonScreen = m_pConfig->readBoolEntry("CommonScreen", _defCommonScreen);
 
     m_numRenderers = m_bDrawBackgroundPerScreen ? TDEApplication::desktop()->numScreens() : 1;
+    if (m_numRenderers < 2) {
+        // Only one screen is currently available; deactivate per-screen rendering but do not overwrite multi-screen settings
+        m_bDrawBackgroundPerScreen = false;
+    }
 
     m_bFinished.resize(m_numRenderers);
     m_bFinished.fill(false);