fix nvgCreateImageFromHandle for GLES2/3.
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; }
Fixed memory leak
Fixed potentially uninitialized local variable warning
Fixed missing void in function with no parameters
Merge pull request #115 from cforfang/small_fixes

Memory leak + small fixes
Removed unused mx,my,len
Working on D3D backend updates
Up to latest NanoVg spec:
- Batched rendering in the flush
- Discard and other refinements to the shaders
- Nearer to the _gl.h header to make it easier to update in future.
Not done:
The non-AA shader (going to combine them)
The demo needs an update pass.