Avoid warping the cursor to the center of the window when not in relative_warp_mode

When relative mode is enabled and not using warp mode, the cursor is
being clipped to the window. Therefore there is no reason to restore the
cursor position to the center.

Avoiding the warp to center simplifies mouse position event flow, as we
are no longer potentially receiving mouse events for the automated
movement of the cursor and can be (mostly) assured that an incoming
event from the windowing system is that of external means.

src/events/SDL_mouse.c

@@ -819,11 +819,10 @@ SDL_SetRelativeMouseMode(SDL_bool enabled)
     mouse->scale_accum_y = 0.0f;
 
     if (enabled && focusWindow) {
-        /* Center it in the focused window to prevent clicks from going through
-         * to background windows.
-         */
         SDL_SetMouseFocus(focusWindow);
-        SDL_WarpMouseInWindow(focusWindow, focusWindow->w/2, focusWindow->h/2);
+
+        if (mouse->relative_mode_warp)
+            SDL_WarpMouseInWindow(focusWindow, focusWindow->w/2, focusWindow->h/2);
     }
 
     if (mouse->focus) {