Fixed interaction between dynamic texture and context.clip()

[SergioRZMasson]

Overview

Allows Dynamic Texture context context.drawImage context.clearRect and context.fillRect to properly work with context.clip.

Implementation

When calling Babylon::Polyfills::Internal::Context::DrawImage the nvgBeginPath is been called every time. This will discard any path that was previously set by the user. The correct behavior should be to not call nvgBeginPath if Babylon::Polyfills::Internal::Context::Clip was previously called.

Also, in Babylon::Polyfills::Internal::Context::Clip the m_rectangleClipping is been used to set the Scissor values even when m_rectangleClipping was never set before (with default values 0, 0, 0, 0). This will make some of the following operations to produce no result. The correct behavior should be to only use m_rectangleClipping if the were set, otherwise use the width and height of the render target.

Finally, when creating Babylon::Polyfills::Internal::NativeCanvasImage the resulting data might be in RGB instead of RGBA. However, during NativeCanvasImage::CreateNVGImageForContext NVG always assumes the format to be RGBA, which might cause a memory out of bounds read if the image was originally RGB. This PR forces bimg::imageParse to always convert the image to RGBA.

More information

Allows the following code to work in Babylon Native: https://playground.babylonjs.com/#FU0ES5#27
Fixes issue #1184

[CedricGuillemet]

LGTM!