Streamlit-drawable-canvas

- Fix background image on Streamlit Cloud and remote servers (thanks andreaferretti)

- New `point` mode (thanks arnauddhaene):
- Adds fixed-radius points to build scatter plots
- Images between frontend and backend are now transferred with URLs computed by Streamlit (thanks kapong)
- Upgrade `streamlit-component-lib` to 1.3.0

- New `polygon` drawing mode (thanks hiankun):
- left-click will add point
- right click will close polygon
- double click will remove latest point
- the Bin button in the toolbar which deletes the canvas content will now empty the history and send back to Streamlit a blank state, even if `update_streamlit` is set to `False`.
- Right-click fires the `send canvas data back to Streamlit` event for all tools (not only the `polygon`) even if `update_streamlit` is set to `False`.

- `initial_drawing` is now used as the initial canvas state. If `None` provided then we create one on the Python side. This provokes the following changes:
- a change in `background_color` will reset the drawing.
- `background_color` will override the background color present in `initial_drawing`.
- if `background_image` is present then `background_color` is removed from `st_canvas` call.
- Upgrade Fabric.js to version 4.4.0.
- Toolbar is now on the bottom left to account for large canvas width.
- Add argument to make the toolbar invisible.
- Make `stroke_width` the minimum size constraint to create a rectangle and circle. Thanks [hiankun](https://github.com/hiankun) for the PR!

- Add `initial_drawing` argument to initialize canvas with an exported canvas state

- Fix state issue with deleting an object through double click