Unified implementation of filters for CloneSources

All filters now derive from FrameBufferFilter, which is always used in a CloneSource. Default FrameBufferFilter is Passthrough filter. Others are Delay and Image filters. Implemented UI selection of filter type, XML session save and load. Linked ImageFilter to Code editor.

Visitor.h

@@ -26,7 +26,6 @@ class Shader;
 class ImageShader;
 class MaskShader;
 class ImageProcessingShader;
-class ImageFilter;
 
 class Source;
 class MediaSource;
@@ -42,6 +41,11 @@ class NetworkSource;
 class MixingGroup;
 class MultiFileSource;
 
+class FrameBufferFilter;
+class PassthroughFilter;
+class DelayFilter;
+class ImageFilter;
+
 class SourceCallback;
 class SetAlpha;
 class SetDepth;
@@ -79,7 +83,6 @@ public:
     virtual void visit (ImageShader&) {}
     virtual void visit (MaskShader&) {}
     virtual void visit (ImageProcessingShader&) {}
-    virtual void visit (ImageFilter&) {}
 
     // utility
     virtual void visit (Stream&) {}
@@ -98,6 +101,11 @@ public:
     virtual void visit (CloneSource&) {}
     virtual void visit (MultiFileSource&) {}
 
+    virtual void visit (FrameBufferFilter&) {}
+    virtual void visit (PassthroughFilter&) {}
+    virtual void visit (DelayFilter&) {}
+    virtual void visit (ImageFilter&) {}
+
     virtual void visit (SourceCallback&) {}
     virtual void visit (SetAlpha&) {}
     virtual void visit (SetDepth&) {}