Make some mouse processors ops optional

Do not force all mouse processors to implement scroll events or touch
events.

app/src/input_manager.c

@@ -657,6 +657,7 @@ input_manager_process_mouse_motion(struct input_manager *im,
                                             im->forward_all_clicks),
     };
 
+    assert(im->mp->ops->process_mouse_motion);
     im->mp->ops->process_mouse_motion(im->mp, &evt);
 
     if (im->vfinger_down) {
@@ -671,6 +672,11 @@ input_manager_process_mouse_motion(struct input_manager *im,
 static void
 input_manager_process_touch(struct input_manager *im,
                             const SDL_TouchFingerEvent *event) {
+    if (!im->mp->ops->process_touch) {
+        // The mouse processor does not support touch events
+        return;
+    }
+
     int dw;
     int dh;
     SDL_GL_GetDrawableSize(im->screen->window, &dw, &dh);
@@ -764,6 +770,7 @@ input_manager_process_mouse_button(struct input_manager *im,
                                             im->forward_all_clicks),
     };
 
+    assert(im->mp->ops->process_mouse_click);
     im->mp->ops->process_mouse_click(im->mp, &evt);
 
     // Pinch-to-zoom simulation.
@@ -795,6 +802,11 @@ input_manager_process_mouse_button(struct input_manager *im,
 static void
 input_manager_process_mouse_wheel(struct input_manager *im,
                                   const SDL_MouseWheelEvent *event) {
+    if (!im->mp->ops->process_mouse_scroll) {
+        // The mouse processor does not support scroll events
+        return;
+    }
+
     // mouse_x and mouse_y are expressed in pixels relative to the window
     int mouse_x;
     int mouse_y;

app/src/trait/mouse_processor.h

@@ -19,18 +19,38 @@ struct sc_mouse_processor {
 };
 
 struct sc_mouse_processor_ops {
+    /**
+     * Process a mouse motion event
+     *
+     * This function is mandatory.
+     */
     void
     (*process_mouse_motion)(struct sc_mouse_processor *mp,
                             const struct sc_mouse_motion_event *event);
 
+    /**
+     * Process a mouse click event
+     *
+     * This function is mandatory.
+     */
     void
     (*process_mouse_click)(struct sc_mouse_processor *mp,
                            const struct sc_mouse_click_event *event);
 
+    /**
+     * Process a mouse scroll event
+     *
+     * This function is optional.
+     */
     void
     (*process_mouse_scroll)(struct sc_mouse_processor *mp,
                             const struct sc_mouse_scroll_event *event);
 
+    /**
+     * Process a touch event
+     *
+     * This function is optional.
+     */
     void
     (*process_touch)(struct sc_mouse_processor *mp,
                      const struct sc_touch_event *event);