This commit fixes the formats as suggested in #299 and fixes stack overflow when using `freetype` backend.

- added support to take screen shots
- added support to render to frame buffer using premultiplied alpha
- fixed spelling NVGaling -> NVGalign

- moved performance counter and drawing to separate file
- tweaks to performance drawing

Needs names for the UI components to distinguish them, and probably stacking vertically for easier comparison.

- moved demo stuff to separate file
- created two separate examples for gl2 and gl3
- initial stab at OGL 3.2 core profile back-end (currently twice as
slows as 2.0)
- API change: glBeginFrame() takes view witdth and height as input
- added FPS counter to demo