Move computing disallowed areas to separate function
Move computing disallowed areas to separate function

The results are checked for intersection in the main update function, and then it crashes because the main function doesn't expect per-extruder disallowed areas yet from the rest.

Contributes to issue CURA-2625.

ynotstartups/Wanhao,hmflash/Cura,fieldOfView/Cura,Curahelper/Cura,hmflash/Cura,ynotstartups/Wanhao,Curahelper/Cura,fieldOfView/Cura