Convert variables to coord_t
Convert variables to coord_t

According to the following review comments:
* https://github.com/Ultimaker/CuraEngine/pull/1364#discussion_r531683148
* https://github.com/Ultimaker/CuraEngine/pull/1364#discussion_r531683874

Contributes to CURA-7678 via CURA-7875

Ultimaker/CuraEngine,Ultimaker/CuraEngine