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Canvas Paint, version 3

We added a second drawing tool : the line drawing tool. It draws 'elastic lines' on a second canvas that has been created on top of the mainCanvas, we call it the "front canvas". By default, we can see through it.
When we draw a line, it is drawn on the front cavans which is cleared each time the mouse moves. This is like - move - draw - clear - that's why we see an "elastic line".
You will notice that the content is deleted each time you draw a new line. Something is missing here !

Pick a tool
line pencil