qxcb: output clear error message when running on an 8-bit display

Currently, running a Qt5 widget app in Xvfb (with its default setup, 8 bit color depth)
outputs a stream of error messages, starting with:
QWidget::paintEngine: QWidget::paintEngine: Should no longer be called

Better output clearly the reason why we end up in that method: painting
on a null image, due to the unsupported image format.

Maybe this should even be a qFatal...

Change-Id: Iae8f1b057518c146bf8e034999c1b5e67ce6ef5e
Reviewed-by: Friedemann Kleint <[email protected]>
Reviewed-by: Samuel Rødal <[email protected]>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -170,7 +170,9 @@ static inline QImage::Format imageFormatForDepth(int depth)
         case 32: return QImage::Format_ARGB32_Premultiplied;
         case 24: return QImage::Format_RGB32;
         case 16: return QImage::Format_RGB16;
-        default: return QImage::Format_Invalid;
+        default:
+                 qWarning("Unsupported screen depth: %d", depth);
+                 return QImage::Format_Invalid;
     }
 }