- 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.

- removed assize from the back-end api
- moved image to paint in renderTriangles call

- fixed off-by-one pixels from demo
- added device-pixel-ratio support (retina support)
- added nvgEndFrame()

- added milliseconds to fps counter
- smooth fps reading a bit more
- changed GL3 to uber shader

- 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

- updated font stash
- added end parameter to text (can draw substring)
- text and text measure return advance/width

- fixed arc rendering for small arcs
- added color wheel to example
- added screenshot