API changed! Fixed slowdown, Shader uses premultiplied colors

- all colors in the shader is handled as premultiplied
- removed alphaBlend param from nvgBeginFrame(), it became obsolete
- added NVGL_TEXTURE_PREMULTIPLIED flag
- separated FBO example
- FBOs are treated as premultipied for correct alpha
- changed FBO storage from depth/stencil 24/8 to stencil 8
- moved discard to end of shader which restores the render speed again
Fix for issue #111

- added nvgQuadTo, draws quadratic bezier curve segment
Fix for issue #108

- fixed stroke cap tessellation amount calculation
Merge pull request #104 from starwing/master

fix nvgCreateImageFromHandle for GLES2/3.
Removed unused mu variable
Changed not used macro from do {} while(0), to for(;;) { ... break; }
Auto detect and resize text altas texture.