A couple of win32u auto-repeat related improvements:
1. Cancel auto-repeat only if the repeat key is released (not other keys). 2. Cancel previous key auto-repeat when starting a new one for a different window (see commit message for more info).
I have kept the changes minimal, which can lead to a few extraneous `kill_system_timer` calls. To improve on this would require more extensive tracking/clearing of `thread_info->key_repeat_msg`.
-- v2: win32u: Cancel previous key auto-repeat when starting a new one.