wayland: Enable relative pointer mode based on the window flag

This can be toggled per-window, so use the individual window flags instead of the global toggle to selectively enable it only for the relevant window in a multi-seat scenario, as is already done with keyboard and pointer grabs.

src/video/wayland/SDL_waylandvideo.h

@@ -96,9 +96,7 @@ struct SDL_VideoData
     int output_count;
     int output_max;
 
-    bool relative_mode_enabled;
     bool display_externally_owned;
-
     bool scale_to_display_enabled;
 };