Provide relative mouse motion vector in event

This will allow the mouse processor to handle relative motion easily.

app/src/input_events.h

@@ -365,6 +365,8 @@ struct sc_mouse_scroll_event {
 
 struct sc_mouse_motion_event {
     struct sc_position position;
+    int32_t xrel;
+    int32_t yrel;
     uint8_t buttons_state; // bitwise-OR of sc_mouse_button values
 };
 

app/src/input_manager.c

@@ -651,6 +651,8 @@ input_manager_process_mouse_motion(struct input_manager *im,
             .point = screen_convert_window_to_frame_coords(im->screen,
                                                            event->x, event->y),
         },
+        .xrel = event->xrel,
+        .yrel = event->yrel,
         .buttons_state =
             sc_mouse_buttons_state_from_sdl(event->state,
                                             im->forward_all_clicks),