[FIX](add extension point) issues arising from marks created with add inheriting functions of marks which have signs; then these would fail evaluation due to not having a proper environment; now signs are created on the fly, when needed.
[FEAT](hide overflow) The new option "leafContentOverflow" allows forcing to hide overflow, independently of the axes' fixedMin and fixedMax properties being set; used by visualizations with custom marks that need to activate clipping of the content panel.