enable png and mp4 output in SimpleOpenGL3App, see

Demos3/SimpleOpenGL3 use commandline parameter
--png_file="pngname" or
--mp4_file="video.mp4"

Thanks to http://blog.mmacklin.com/2013/06/11/real-time-video-capture-with-ffmpeg/

btgui/OpenGLWindow/SimpleOpenGL3App.h

@@ -38,6 +38,9 @@ struct SimpleOpenGL3App
 	int	registerCubeShape(float halfExtentsX=1.f,float halfExtentsY=1.f, float halfExtentsZ = 1.f);
 	int	registerGraphicsSphereShape(float radius, bool usePointSprites=true, int largeSphereThreshold=100, int mediumSphereThreshold=10);
 
+    void dumpNextFrameToPng(const char* pngFilename);
+    void dumpFramesToVideo(const char* mp4Filename);
+
 	void drawGrid(DrawGridData data=DrawGridData());
 	void swapBuffer();
 	void drawText( const char* txt, int posX, int posY);