This commit fixes #300 and #306 that “control may reach end of non-void function” error occurs in `glnvg_convertBlendFuncFactor()`. The function now returns `GL_INVALID_ENUM` for invalid blend factor and if that happens, `glnvg__blendCompositeOperation()` uses default source-over composite operation.

Fixes the kerning value for freetype.

This commit fixes issue #302 that the kerning value returned by freetype is not converted to integer properly.

Support fallback fonts (code refactoring for #299)

This commit fixes the formats as suggested in #299 and fixes stack overflow when using `freetype` backend.

Implements composite operation for blending renderings.

This commit updates the implmentation of global composite operation to follow the advices described at c4b865796d (r74248556)

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.