PolygonGenerator: Finished main implementation

It's been 84 years, but the polygon generator not only works, but now has bezier support. The UI could still use some work, but this was the hardest part. I guess complex masking should be possible now, if perhaps slightly cumbersome until the UI is smoothed out.

app/node/inputdragger.h

@@ -46,6 +46,11 @@ public:
     return input_being_dragged;
   }
 
+  const QVariant &GetStartValue() const
+  {
+    return start_value_;
+  }
+
 private:
   NodeKeyframeTrackReference input_;