fix: Return gl context in getWebGLContext if available
fix: Return gl context in getWebGLContext if available

* throws an error instead of console.error when WebGL is not supported
  otherwise this.gl is undefined and the errors would be pretty confusing for
  users in either case)
* WebGLRenderer#getContext returns the previously retrieved GL context if
  available

talves/famous-engine,talves/famous-engine