fix for event lag

* moves glfwPollEvents to top of App loop, and sleep logic to after display update, so that UI events can be processed immediately
ofImage saveImage: only swapRGB on required formats. Closes #3229

Also when swapping RGB reflect on the internal pixel type in ofPixels
Merge branch 'master' of github.com:openframeworks/openFrameworks
ofGstVideoPlayer: add getters for missing private varibles. Closes #3232
ofVideoPlayer: remove extra method declaration
ofPixels: avoid warnings
ofImage: use correct formats when loading
Merge pull request #3226 from tgfrerer/test-pollevents-snappyness

possible fix for event lag
Merge branch 'master' of https://github.com/openframeworks/openFrameworks.git into rexgo