This commit updates NanoVG's bgfx backend to match the original NanoVG's latest commit, which has significant changes about composite operation implementation as mentioned in pull request https://github.com/memononen/nanovg/pull/312.
This commit updates bgfx's NanoVG codebase to adapt the latest NanoVG commit since I've implemented blending support for the original NanoVG and there is no need of `nvgState()` anymore.
This commit fixes the bug that the bgfx does not respect the `devicePixelRatio` parameter passed to nanovg's `nvgBeginFrame()` function. The problem is caused by the `bgfx::setViewRect()` call defined in the `nvgRenderViewport()` function, as it should take size in consideration of the `devicePixelRatio` value.
However, this commit does not fix nanovg's example app because currently there is no easy way to pass the scale value to the `ExampleNanoVG::update()` method as it calls `nvgBeginFrame()`.
