Great tool and very quick!
Just reporting one tiny bug - it seems if you draw a circle then a square, you get a persistent red circle that can't be drawn over with the square.
One other thing - would it be possible to add shift/CTRL clicking to shape drawing like photoshop for more control - for example with the rectangle :
Mouse only - Upper left corner starts and cursor click, bottom right moves with cursor while button down and is completed on release.
CTRL - The above but drawing centred on cursor.
SHIFT - Constrain to cube shape (equal sides)