SEGZONEs are no longer created by pcbnew but may exist in older boards.
This allows deleting the old fills in a manner similar to the deletion
in Legacy canvas.
Fixes: lp:1794571
* https://bugs.launchpad.net/kicad/+bug/1794571
This is a boyscouting commit to standardize the threading of zone fills.
We do not need to join threads after their completion, instead we simply
allow them to clean up their memory without blocking the user. This
also sets the maximum number of threads that may be created to the
number of zones being filled. More than this will only leave un-used
threads being created and immediately killed.
We also include the connectivity search as a phase in the fill progress
reporter. This was the case before but did not utilize the correct
maxsize, leading to stalled progress bar.
Jeff Young
Seth Hillbrand
jean-pierre charras
Tomasz Włostowski