Deleting/cutting causes position of floated pixels to jump on first mouse motion

With rx 0.4.0, when I...

1. Press `v`
2. Draw a selection box from top-left to bottom-right (as is my habit)
3. Press `d` to cut the selection

...as soon as I move my cursor, the selected block of pixels will jump so that the top-left pixel is under the cursor. This is a big inconvenience when I just want to move a block of pixels up/down/left/right a pixel or two relative to their original position because my habit of selecting from top-left to bottom-right invariably puts the cursor over the bottom-right pixel of the selection.

`d` should either grab the floated selection based on where the cursor was at the time `d` was pressed, relative to the selection or warp the pointer to the top-left corner of the floated selection.