many callers of setDesignationAt simply didn't bother with the priority
parameter. change the default value of the priority param so that by
default we will keep the previous value instead of overwriting it
The map_block->designation.{dig,smooth} are reset to zeros when a job
posting is created for the designation. The job is then used to override
the designation state in the map_block. To make the new designation set
propogate to jobs the job structure would require updating. The update
would be possible a complex operation. The simple alternative is to
remove the job and let df create a new job in the next tick.
Fixes#1229
Unlike changevein, which changes the material in an existing vein
object, with this feature you can set the material for individual
tiles, and the cache will write it out as vein objects.
The array inside the cache data is changed from layer material to
vein material, because layer material is easy to compute.