When I worked on some bugs I got a situation
when I tried to change the Region object via a different set methods.
But these changes are ignored because the reference was to
ImmutableRegion which replaces all setter to no-op methods.

In the fix I propose to remove specific ImmutableRegion class, and make
the whole Region class immutable:
  - setXX methods are removed, since most of them are unused.
  - the new getInstance(int box[], SpanIterator) was added, so the
appendSpans() can be changed to the private.
  - small cleanup in equals, toString.

http://mail.openjdk.java.net/pipermail/2d-dev/2016-May/006838.html