Render loop lost track of which blocks had been rendered due to a
synchronization issue. Specifically, std::vector<bool> is special-cased
as a bitfield and so is pretty much guaranteed not to be atomic unless
synchronized externally.
Also:
- Clean up types of a few variables
- Clean up openmp sharing type of variables (may result in better
optimization)
- Replace shared rendered block count with an openmp reduction (results
in fewer locks)
Fixes: lp:1608289
* https://bugs.launchpad.net/kicad/+bug/1608289
82ed0fde32