Fix Layer_Shape: Invert parameter didn't work right.
Fix some missing cairo_save and cairo_restore on some layers
ConicalGradient: add support for Cairo render using cairo_t*.
Fix SpiralGradient Cairo render using cairo_t*
CurveGradient: add support for Cairo render using cairo_t*.
Revert Cairo Overlay blend method to its old definition.
Fix missing displacement in Cairo render for ConicalGradient