In GimpTool, track the last-seen pointer coordinates, modifier
state, and event time, during button_press() and motion() events
and use those to synthesize a button_release() event when
comitting/halting the tool, if the tool is still active, and a
matching button_release() event has not been received.

The paint tools (as well as other tools) require each
button_press() event to be matched by a button_release() event in
order to properly finish their operation, but one isn't organically
generated when switching tools due to a device change.