WebGLCanvas doesn't support for WebGL 2 or 3

[CZakarya]

When you request "webgl2" from a canvas (canvas.getContext("webgl2")) it returns a null object. (see line 53 of src/webgl/canvas.ts). ThreeJS no longer supports WebGL 1 as of version r163. As of writing, the latest version is r169. In order to use modern ThreeJS with this project's WebGLCanvas, it needs to support WebGL2. PR #1 seems to add WebGL2 support, so that may be able to be used as a reference or could be merged.