touch: Fix panic with -Zconvert-mouse-to-touch (#36531)

- We previously converted all mouse move events to touch events, but we
should only be doing that while a mouse button is pressed (a finger
always does touch-down -> move -> up / cancel)
- Only consider Left mouse button for mouse-to-touch. We currently
already hardcode TouchId 0, which we would need to change in order to
properly support Multi-touch. Since we don't have any Multi-touch
gestures at the moment, we leave this unimplemented and simply only
evaluate the left mouse button.

Testing:  Manual testing. We don't have servodriver support yet.
Fixes: #36526

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/compositing/touch.rs

@@ -282,6 +282,10 @@ impl TouchHandler {
         debug_assert!(old.is_some(), "Sequence already removed?");
     }
 
+    pub fn try_get_current_touch_sequence(&self) -> Option<&TouchSequenceInfo> {
+        self.touch_sequence_map.get(&self.current_sequence_id)
+    }
+
     pub fn get_current_touch_sequence_mut(&mut self) -> &mut TouchSequenceInfo {
         self.touch_sequence_map
             .get_mut(&self.current_sequence_id)