refactor to have a single global current brush on Canvas::m_current_brush and make panels read from it for drawing instead of relying on their own copy

engine/app_events.cpp

@@ -81,7 +81,7 @@ bool App::mouse_scroll(float x, float y, float delta)
     MouseEvent e;
     e.m_type = kEventType::MouseScroll;
     e.m_pos = { x / zoom, y / zoom };
-    e.m_scroll_delta = delta * 0.1f;
+    e.m_scroll_delta = delta;
     auto ret = layout[main_id]->on_event(&e);
     layout[main_id]->update();
     return ret == kEventResult::Consumed;