Removed 'canvas' property from 'IWebGL2RenderingContext'

[DanielRosenwasser]

I tried building xterm.js recently, and got the following error:

addons/xterm-addon-webgl/src/GlyphRenderer.ts:305:26 - error TS2339: Property 'canvas' does not exist on type 'IWebGL2RenderingContext'.

305     gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);   

Looks like the canvas property was removed as per #1029, but was that intentional? MSDN says

WebGL 2 is an extension to WebGL 1. The WebGL2RenderingContext interface implements all members of the WebGLRenderingContext interface. Some methods of the WebGL 1 context can accept additional values when used in a WebGL 2 context. You will find this info noted on the WebGL 1 reference pages.

[saschanaz]

No, it was accidental. I'll take a look.