Creates a canvas for recording graphical operations into the given picture recorder.
Graphical operations that affect pixels entirely outside the given
cullRect might be discarded by the implementation. However, the
implementation might draw outside these bounds if, for example, a command
draws partially inside and outside the
cullRect. To ensure that pixels
outside a given region are discarded, consider using a clipRect. The
cullRect is optional; by default, all operations are kept.
To end the recording, call PictureRecorder.endRecording on the given recorder.
factory Canvas(PictureRecorder recorder, [ Rect? cullRect ]) = _NativeCanvas;