pixelsampler: added pixel sampler panel

When visible, this panel will show the current reference and display transformed
color under the cursor of any viewer.

app/codec/frame.h

@@ -25,6 +25,7 @@
 #include <QVector>
 
 #include "common/rational.h"
+#include "render/color.h"
 #include "render/pixelformat.h"
 #include "render/videoparams.h"
 
@@ -48,6 +49,9 @@ public:
   const int& height() const;
   const PixelFormat::Format& format() const;
 
+  Color get_pixel(int x, int y) const;
+  bool contains_pixel(int x, int y) const;
+
   const rational& sample_aspect_ratio() const;
   void set_sample_aspect_ratio(const rational& sample_aspect_ratio);