Add hint to disable mouse warping when calling SDL_SetWindowFullscreen

[edo9300]

Currently, both in sdl2 and sdl3, when the window is made borderless fullscreen, sdl will warp the mouse pointer to keep it relative to the window 0,0 coordinates. This behaviour really seems a vestigial leftover from when exclusive fullscreen was common with associated resolution change, but now it will just make the mouse jump around the window when it's made fullscreen.
It would be nice to have a hint to disable this behaviour (even nicer if for both sdl versions).
On sdl3 it could even make more sense to have this default off and make the hint enable the behaviour, as i don't really think it has many uses nowadays (especially with multi monitor setups, where you coudl even end up having the cursor jump across monitors), and also will streamline the behaviour with the wayland/xwayland backends after this commit b1e01b9