slice positions are zero-based (this fixes problems when editing a slice)

--HG--
branch : 0.9
extra : rebase_source : c1af520d6f7221baee01b0261a68699a0ac59476
be consistent with the slice renderer interface (this also allows the backend to inject more context data, like the slot and stuff)

--HG--
branch : 0.9
extra : rebase_source : 365a84951fa4f898b4b1e564b84c235ee97360c1