Further clean up of the `me.video` API

* removed `me.video.createCanvasSurface()` that was deprecated, and not
brining any real benefits

* added a `me.video.getContext2d' function, that will return the
context object of the specified canvas object, and will take care of
enabling/disabling antialiasing feature if required (whole library has
been updated to now uses this function when required)