Merge #5678

5678: Reset selected quads when switching layers (fixes #5673) r=Jupeyy a=def-

<!-- What is the motivation for the changes of this pull request -->

## Checklist

- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: def <dennis@felsin9.de>

src/game/editor/editor.cpp

@@ -697,6 +697,7 @@ void CEditor::SelectLayer(int LayerIndex, int GroupIndex)
 		m_SelectedGroup = GroupIndex;
 
 	m_vSelectedLayers.clear();
+	m_vSelectedQuads.clear();
 	AddSelectedLayer(LayerIndex);
 }