Gradient tool always re-selects its end point after any click/drag

[0HyperCube]

Reproduce:

• Draw gradient
• Try and select a non-end point (i.e. the start or middle)

broken_gradient_selection.mp4

This is because in the (GradientToolFsmState::Drawing, GradientToolMessage::PointerUp) FSM transition, the SelectedGradient::new is called, which sets the selected point to the end one. A regression introduced by #1945.

[Sidharth-Singh10]

#2472 fixes this