Add relative mode flag to mouse processors

The default mouse injection works in absolute mode: it forwards clicks
at a specific position on screen.

To support HID mouse, add a flag to indicate that the mouse processor
works in relative mode: it forwards mouse motion vectors, without any
absolute reference to the screen.

app/src/input_manager.c

@@ -659,7 +659,11 @@ input_manager_process_mouse_motion(struct input_manager *im,
     assert(im->mp->ops->process_mouse_motion);
     im->mp->ops->process_mouse_motion(im->mp, &evt);
 
+    // vfinger must never be used in relative mode
+    assert(!im->mp->relative_mode || !im->vfinger_down);
+
     if (im->vfinger_down) {
+        assert(!im->mp->relative_mode); // assert one more time
         struct sc_point mouse =
             screen_convert_window_to_frame_coords(im->screen, event->x,
                                                   event->y);
@@ -772,6 +776,12 @@ input_manager_process_mouse_button(struct input_manager *im,
     assert(im->mp->ops->process_mouse_click);
     im->mp->ops->process_mouse_click(im->mp, &evt);
 
+    if (im->mp->relative_mode) {
+        assert(!im->vfinger_down); // vfinger must not be used in relative mode
+        // No pinch-to-zoom simulation
+        return;
+    }
+
     // Pinch-to-zoom simulation.
     //
     // If Ctrl is hold when the left-click button is pressed, then

app/src/mouse_inject.c

@@ -151,4 +151,6 @@ sc_mouse_inject_init(struct sc_mouse_inject *mi,
     };
 
     mi->mouse_processor.ops = &ops;
+
+    mi->mouse_processor.relative_mode = false;
 }

app/src/trait/mouse_processor.h

@@ -16,6 +16,13 @@
  */
 struct sc_mouse_processor {
     const struct sc_mouse_processor_ops *ops;
+
+    /**
+     * If set, the mouse processor works in relative mode (the absolute
+     * position is irrelevant). In particular, it indicates that the mouse
+     * pointer must be "captured" by the UI.
+     */
+    bool relative_mode;
 };
 
 struct sc_mouse_processor_ops {