The main change is on `ViewBox.getContextMenus`, which now returns an
up-to-date of actions that `GraphicsScene.addParentContextMenus` can use.
Also, `getContextMenus` was given a default implementation in the base
class (falling back on `getMenu` if defined), and some cleanup was done.
- ROI updates on sigTransformChanged
- ViewBox is more careful about accepting all auto-range changes up to the point it is disabled, even if the auto-range calculation is deferred.
- GraphicsItem.pixelVectors copies cached results before returning
- Multiprocess fixes for Windows:
- mmap/shm uses anonymous maps rather than tempfiles
- avoid use of getppid and setpgrp
- work around hmac authentication bug (use os.urandom to generate key)
