v1: Canvas buffered shapes (recording), `Image` shape

1. Add an ability to "burn" certain shapes into internal buffer, so once drawn they are displayed even if removed from `canvas.shapes` list.
2. Add `canvas.clear_capture()` method to delete buffered content, i.e. clean canvas.
3. Add `canvas.get_capture()` method to return recorder image in PNG format.
4. Add `Image` shape to call `canvas.drawImage` in Flutter.