Rework LayerData mesh generation for improved performance
Rework LayerData mesh generation for improved performance

Rather than using a lot of calls to MeshData::addVertices which is very
slow, pre-allocate numpy arrays, fill them with data from the polygons
and then assign them to the mesh data's arrays. This greatly increases
performance of the layer data calculation.

Contributes to #52

bq/Ultimaker-Cura,hmflash/Cura,fieldOfView/Cura,ynotstartups/Wanhao,senttech/Cura,quillford/Cura,markwal/Cura,fxtentacle/Cura,Curahelper/Cura,markwal/Cura,lo0ol/Ultimaker-Cura,senttech/Cura,ynotstartups/Wanhao,DeskboxBrazil/Cura,quillford/Cura,fieldOfView/Cura,totalretribution/Cura,ad1217/Cura,hmflash/Cura,totalretribution/Cura,lo0ol/Ultimaker-Cura,bq/Ultimaker-Cura,DeskboxBrazil/Cura,Curahelper/Cura,fxtentacle/Cura,ad1217/Cura