Fixes for GLES2
GLES3
Merge pull request #17 from andrewcorrigan/master

GLES3
Removed scaling from nvgTextBounds and nvgVertMetrics and commented the behavior
Tweaks to GL ES

- use nvgCreateGLES2() instead of nvgCreateGL2() when NANOVG_GLES2 is
specified (ditto for gl3)
- use GL_RED instead of GL_LUMINANCE in ES3
Added retina/hi-dpi support

- fixed off-by-one pixels from demo
- added device-pixel-ratio support (retina support)
- added nvgEndFrame()
Use GL_R8 instead of GL_RED on ES3
NANOVG_GLES2_IMPLEMENTATION implies NANOVG_GLES2
Merge pull request #19 from andrewcorrigan/master

NANOVG_GLES2_IMPLEMENTATION implies NANOVG_GLES2
stdio.h
suppresses "explicitly assigning a variable to itself warning" and potential unused parameter warnings
Added float cast to viewport size

Added float cast to remove compiler warnings.
Merge pull request #20 from andrewcorrigan/master

stdio.h
gl2 size cast
Merge pull request #22 from devnought/cwarn

Added float cast to viewport size
Fixed unused variables and scissor under zoom

- added NVG_NOTUSED to suppress unused param warnings
- fixed scissor anti-aliased border width when zoomed
Simplified back-end API a little

- removed assize from the back-end api
- moved image to paint in renderTriangles call
Added experimental GL3 back end which buffers whole frame