To be consistent with other wlr_xdg_* structs,
wlr_xdg_positioner_resource is renamed to wlr_xdg_positioner and made
public, and wlr_xdg_positioner is renamed to wlr_xdg_positioner_rules.
Functions which operated on wlr_xdg_positioner were renamed and updated
accordingly.
When calling assert(0) instead of returning a value, -Wreturn-type
warnings are triggered because assertions can be disabled. Replace these
assertions with abort().