Fix bug where switching to mouse steering uses old now-invalid analog numbers

[aggregate1166877]

Describe the bug
Under certain conditions, the ship steering will get flung in some direction when entering locked mouse steering.

Steps to reproduce
Steps to reproduce the behavior:

1. Ensure hyperdrive is active.
2. Middle click to enter mouse steering.
3. Move in some direction.
4. Middle click to deactivate mouse steering.
5. Middle click again.
6. Ship is flung in the last direction you were steering.

Expected behavior
Steering should be centered when entering mouse steering.

Desktop (please complete the following information):

• OS: Windows
• Build 72

[aggregate1166877]

Bug (accidentally) fixed while implementing #44, which involved a rewrite of how mouse input is handled.

Closing issue as this will be merged in the next PR.