Currently working with gl3buf backend.

- changed tessellation threshold from 0.3 to 0.25
- fixed scissor AA on retina
- fixed stroke AA on retina

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

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

- added NVG_NOTUSED to suppress unused param warnings
- fixed scissor anti-aliased border width when zoomed

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

- added option to turn off AA fringe generation
- added examples which use no AA fringes and  MSAA

- set swap interval to 0 to better see performance
- draw convex shapes without stencil to speed things up
- improve vertex buffer upload for gl3

- 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

- fixed nvgArcTo
- added nvgTransRGBA, allows to change alpha of color
- added nvgFindFont, allows to find font id by name

- 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