[Feature] Lock/Unlock Mouse Position

Afaik, currently we have no feature that allows locking/unlocking mouse position for things like free camera rotation and gui control objects like boundless sliders. The variety of such operations and needs for controls are so that it's the best to have a dedicated function to control that behaviour, not tied to Spring's hotkey system. I assume, this would approve existing COFC code as well, that uses a workaround which I tested to be imperfect in terms of accuracy for a case of low FPS (a workaround including WarpMouse to make the cursor stay still, apparently is the reason of a non-negligible inaccuracy in my tests).
The proposition is to add 2 functions:
- LockMousePosition(locked)
- GetLastUpdateMouseMovement() -> dx,dy

All in all, this would save from quite cumbersome code with the mouse warps forth and back, as well as give an accurate behaviour.

P.S. _Potentially I may come up with a PR myself in a few months (no good window to take on engine work for now)._

Start by locating the existing COFC mouse-position workaround and its uses of WarpMouse, then review how mouse movement is exposed by the engine. Done means providing dedicated lock/unlock behavior and accurate last-update movement values without relying on the hotkey system or repeated warps; validate behavior under low FPS as described in the issue.

Written by the indexing model from the issue text.