Currently working with gl3buf backend.

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

Removed some unused variables to silence compiler warnings, initialized
variables to 0, added varialbe casts to silence additional compiler
warnings.

- merged font stash optional FreeType support

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

- added better logic for inner bevels
- tweaks to reduce overdraw at edges
- fixed convex shape logic

- implemented missing scissor from draw triangles
- buffer a whole frame of uniforms
- moved paint to uniform conversion earlier
- simplified render loops and data structures
- renamed bunch of things
- combined shaders, use define to disable AA

- correct fix for ubo memroy allocation (use returned alignment instead
of struct size)
- separated convex fill to own command type
- moved uboPos to call struct

Cast void* to char* to remove compiler warning in C++ project in VS2013

- 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 remove compiler warnings.

- 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

- 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