Selection tool and GUI update

Hi,

Using the rectangle selection tool, I noticed something different in GUI.

1 - Open an image in ImageJ
2 - Select "rectangle selection tool"
3 - Press "Shift" to create a square selection. Pick the first point, drag, and released the mouse and "Shift".
4 - Do process 3 again in other points.
5 - After process 4, the 2 selections will join
6 - Dragging a selection, you can see that the image in the GUI is not updated correctly. The fact that does not occur when there is only one selection (without a union).

This glitch does not occur with an oval selection tool. I suggest adopting the same implementation and tests.

Thank you all

Start by reproducing the issue in ImageJ with the rectangle selection tool: create two square selections with Shift, join them, and drag the result. Compare this behavior with the oval selection tool; done means the GUI updates correctly after dragging a joined selection and tests cover the regression.

Written by the indexing model from the issue text.