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/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;
 }