This commit implements the `nvgGlobalCompositeOperation()` function to support blending between frames. All operations defined in HTML5 canvas API are supported. Also, it is possible to create custom composite operation by calling `nvgBlendFunc()` or `nvgBlendFuncSeparate()` functions.

Add support for bgfx

This commit updates the `nvgBeginFrame()` function to forward the received `devicePixelRatio` parameter to the `renderViewport()` function so it is possible to configure the environemnt correctly when using `bgfx` backend.

Fixing clang compiler warnings.

Fix typo in nvgCreateFontMem() description

- nvglImageHandle was changes to contain GL postfix, changed GL utils
to use that too
- fixed silly failure check bug in FBO demo

Fixed fontstash.h crashing

Fix for issue #172

fixed type usage

in some situations int and GLsizei is not the same type which leads to conversion warnings

Improve the use of NVGLUframebuffer.

The `NVGLUframebuffer` structure contains a `ctx` member but was actually never used. This commit takes advantage of the `ctx` property so there is no need to keep the reference of the context elsewhere after calling `nvgluCreateFramebuffer()`, or it would add difficulty for maintenance. As a result, the `nvgluDeleteFramebuffer()` function only needs to take one `NVGLUframebuffer` instance as the parameter.

Fix spelling mistake in readme

Fix typos.

- fixed stroke to use lino join style to calculate required vertex
count

Line break would duplicate a word if the first character of a word
would cause a line break

Added check in addPoint to avoid duplicated moveTos breaking

Redundant GL state filter

It guards addPoint from "stealing" the point from the previous path if they're the same, which makes it break when you do something like this:
```
moveTo(50, 50)
moveTo(50, 50)
lineTo(100, 100)
```

Before the fix there's no line shown. With the second moveTo, there's a second path added, but without the 50,50 point, because it was already added with the first moveTo. This makes the second path only contain the point 100,100 so it doesn't draw at all.

After the fix, the point equality check is ignored for the first point in the path, so the second path gets the 50,50 point too and that makes it behave as expected.

This is only one solution to the duplicated moveTo problem and it's likely not the best one, but it's one that's easy to add and it seems to work just fine.

Fixes nvgluCreateFramebuffer().

The framebuffer and renderbuffer did restore in `nvgluCreateFramebuffer()` on succeed. However, they are not restored if error occurs. This commit fixes it.

Fixed compile error occuring if FONS_USE_FREETYPE is defined.

Consistent code syntax and typo fix

Fixed syntax error of checking defined identifier

The syntax is wrong when checking defined identifier.

Added nvgCancelFrame()

This commit implements a new nvgCancelFrame() function for the purpose described in #168.

Currently, if calling nvgBeginFrame(), the nvgEndFrame() function must be called to finish the drawing. However, if knowing the drawing would take too much time and want to give up the drawing. There is no way to cancel it. nvgCancelFrame() gives another choice to end the frame without actually drawing.

Merge uniforms into a single uniform array

Add FBO support for GLES

Fix for issue #162

To support FBO for iOS, the current framebuffer and renderbuffer must be restored after calling nvgluCreateFramebuffer(). The framebuffer should also be restored when calling nvgluBindFramebuffer(NULL) as well.

Conflicts:
	src/nanovg_gl.h

- API changed!
- moved image repeat from nvgImagePattern() into image creation
nvgCreateImage*()
- made flip-y and premultiplied common image flags (not just GL)
- removed nvglImageFlags(), flags passed in via
nvglCreateImageFromHandle() flags
- nvgluCreateFramebuffer takes image flags as param

- added NVG_DEBUG which can be used to omit calls to glGetError()
- removed return value for glnvg__checkError
- create flag defines to enum

added new graph style

Fix texture completeness for non-POT textures on GLES2/WebGL