Instead of showing what clicking the button will do, display the
current state, which can be toggled by clicking the button.

While at it, refactor some code and make sure all UI hints are
cleared when the mouse leaves the map area.

Add a "UI hints" canvas on top of the overlay layer to draw UI
hints. For now, only use it in the rectangle tool to show how big
the rectangle is going to be.

Now that it's easier to see the exact points in the overlay thanks
to the crosshair cursor, it's important to reset the line width to
1 pixel when drawing rectangles to that only the expected area is
covered/uncovered.

It seems to only/mostly appear in Firefox, but it might be a
general issue. And in any case, it seems more correct this way.