Set WebAssembly window background transparent
Rendering a WebGL view that blends with the HTML background is common practice and makes great web app integration (see video below).
This MR makes the WebGL view handle the alpha channel if pixels are not completely opaque.
If the user wants to see the background, it's their responsibility to make sure the background is opaque.
The premultipliedAlpha option is disabled to have a correct blending result.
Edited by Michael Migliore