Win32 modification to get code to run, and performance increase for buffered gl3

STBI_NOTUSED

- 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

Added float cast to viewport size

stdio.h

Added float cast to remove compiler warnings.

NANOVG_GLES2_IMPLEMENTATION implies NANOVG_GLES2

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

- use nvgCreateGLES2() instead of nvgCreateGL2() when NANOVG_GLES2 is
specified (ditto for gl3)
- use GL_RED instead of GL_LUMINANCE in ES3

GLES3

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

- merged GL ES2 and GL2 backends
- GL2 uses now same structure as GL3

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

- link to framework CoreVideo on OSX, fixes compiling on OSX 10.9

- 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

MSVC compile fix (example); win32 screenshot added

- quantize font rendering scale
- round kerning values for less flicker

fix for https://github.com/memononen/nanovg/issues/3

- 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