Player display of non-playable source

Show also non-playable sources to allow testing pre- post-display. Show post-processed image only if source made changes.

Source.h

@@ -194,6 +194,7 @@ public:
     // get properties
     float depth () const;
     float alpha () const;
+    bool  textureTransformed () const;
 
     // groups for mixing
     MixingGroup *mixingGroup() const { return mixinggroup_; }