Forward mouse motion only on main clicks

Mouse motion events were forwarded as soon as any mouse button was
pressed.

Instead, only consider left-click (and also middle-click and right-click
if --forward-all-clicks is enabled).

app/src/input_manager.c

@@ -595,8 +595,12 @@ convert_mouse_motion(const SDL_MouseMotionEvent *from, struct screen *screen,
 static void
 input_manager_process_mouse_motion(struct input_manager *im,
                                    const SDL_MouseMotionEvent *event) {
-    if (!event->state) {
-        // do not send motion events when no button is pressed
+    uint32_t mask = SDL_BUTTON_LMASK;
+    if (im->forward_all_clicks) {
+        mask |= SDL_BUTTON_MMASK | SDL_BUTTON_RMASK;
+    }
+    if (!(event->state & mask)) {
+        // do not send motion events when no click is pressed
         return;
     }
     if (event->which == SDL_TOUCH_MOUSEID) {