Erase support (transparent background only)

[tmarzeion]

Here is PR with code change that allows user to set erase mode.
Set controller.eraseMode = true for erase mode.

Note: This works only for transparent background. If You want this to work with colored background, further code changes are needed. My suggestion is moving background to another canvas, by simply using Stack and inserting simple box view below painter layer.

[EP-u-NW]

Thanks for your work :)
I will merge it now since it seems valid code, but will wait with publishing to pub until I do some more tests. You could expect it to reach pub in about two days.

[EP-u-NW]

So, I published everything to pub.
Based on your work I was also able to extend the erase mode to erase on none transparent background.
Greate work, thanks again 👍