Revert getWebGLContext => getContext change

This change allowed TWGL to create a WebGL 2 context when available and
fall back to WebGL 1 when that's the only option. This should in theory
be harmless, but deserves more extensive testing before rolling it out.

src/RenderWebGL.js

@@ -80,7 +80,7 @@ class RenderWebGL extends EventEmitter {
      * @private
      */
     static _getContext (canvas) {
-        return twgl.getContext(canvas, {alpha: false, stencil: true});
+        return twgl.getWebGLContext(canvas, {alpha: false, stencil: true});
     }
 
     /**