One goal is to make the code for drawing with the Ink tool similar to the one to free highlighting:
it doesn't really make sense to have so different ways to do almost the same thing.
When the zoom level is high, it'll avoid to create a too big canvas covering all the page which consume
more memory, makes the drawing very slow and the overall user xp pretty bad.
A second goal is to be able to easily implement more drawing tools where we would just have to implement
how to draw from the pointer coordinates.
When an highlight is self-intersecting, the outline was drawn inside.
In order to remove it, we use a svg mask to exclude the shape inside
when drawing the outlines.
That leads to change the outline 1px,white-2px,blue-1px,white to a
2px,white-2px,blue: the part of the stroke which is inside the shape
is removed because of the mask.
The free highlighting is enabled when the mouse pointer isn't on some text.
Then we draw a shape with smoothed borders corresponding to the movement of
the mouse.
Printing/saving and changing the thickness will come later.
The goal is to be able to get these outlines to fill the shape corresponding
to a text selection in order to highlight some text contents.
The outlines will be used either to show selected/hovered highlights.
