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()