Added float cast to viewport size

Added float cast to remove compiler warnings.
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
Merge pull request #23 from andrewcorrigan/master

STBI_NOTUSED