WebGL support

## Feature

Recently I got stumbled with the [node-wpe-webgl](https://github.com/WebPlatformForEmbedded/node-wpe-webgl) repo, that implements a `webgl` compatible rendering context for Node.js. It's mostly a translation of the `webgl` APIs to the ones of EGL, but code is very simple and mostly copy-and-paste. I was thinking it would be possible to use it to provide support for the `webgl` rendering context in `node-canvas`, both directly by delegaing to it, or picking the code and integrating it directly into `node-canvas` code. First option would be very simple and straigforward, but has the problem that only would work with screen-based backends (in memory rendering would be possible by using an off-screen rendering surface, but would need a bit more work), while the second one would allow to have it integrated for all backends and also use the same rendering surface (at the cost of having Mesa as another dependency).