Always apply queued movement, even when there's no duration. Fixes crash with --start="bal isra".

When a script disables a reference that still has movement queued, trying to apply that movement will then fail due to the reference not being in the scene. Therefore, we should make sure that movement is always applied in the frame that it's queued in.
Bring back the option to not grab mouse. Useful if running in a mouse-controlled GUI debugger.