streamline cursor clipping logic on windows

[expikr]

This commit does the following:

• add logic in the WM_MOUSEMOVE case of the Window to conditionally call WIN_UpdateClipCursor upon receiving cursor motion if SDL is expecting the mouse to be clipped in some way (Fixes Cursor not constrained to window when using SDL_SetRelativeMouseMode and another game is running #7890)
• remove Windows-specific periodic refresh of cursor clipping and its SDL_HINT_MOUSE_RELATIVE_CLIP_INTERVAL hint (superceded by the above bullet point)
• streamline the processing logic within WIN_UpdateClipCursor for better readability of each branch, and avoid calling the Platform API until it is absolutely necessary.
• move relative_mouse_center field from Windows-specific per-window SDL_WindowData to the global SDL_Mouse struct, and the corresponding hint callbacks to SDL_mouse.c instead of SDL_windowswindow.c

relevant commits:

e56f05b (apr 2024)
6c96217 (mar 2024)
ab5351f (mar 2024)
50203d5 (dec 2020)
44f50c6 (jun 2020)
55b24b9 (sep 2018)