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/

build3/premake4.lua

@@ -116,7 +116,7 @@ if findOpenGL() then
 	include "../btgui/OpenGLWindow"
 
 --	include "../Demos3/ImplicitCloth"
---	include "../Demos3/SimpleOpenGL3"
+	include "../Demos3/SimpleOpenGL3"
 
 	include "../btgui/lua-5.2.3"
 	include "../test/lua"