Add a USE_WEBGL2 pref.

"try to grab" makes it sound like perhaps it would attempt to initialize a WebGL2 context, and if not available, revert to WebGL1? The code does not do that, it so perhaps this comment should explicitly reflect that, e.g. "This mode will require a WebGL2 context if no version is specified." or similar?

Also the line 719 in src/library_gl.js: `Module.print('Could not create canvas: ' + [errorInfo, e]);` would be good to be updated to also print which WebGL version was attempted: "Could not initialize WebGL 1/2 context on a canvas: "