Merge pull request #124 from andrewcorrigan/master

allows for nanovg_gl prototypes to be declared simultaneously
Auto detect and resize text atlas texture.
Merge pull request #123 from starwing/master

Auto detect and resize text altas texture.
update upstream stb things.
Merge pull request #125 from starwing/master

update upstream stb things.
Fix for issue #127

- set stb_image flags so that it loads correct data in iPhone
only specifies precision for fragment shader, vertex shader defaults to highp as per gles2/gles3 spec