RFR: JDK-8199216: Memory leak and quadratic layout time with nested nodes (hbox) and pseudo-class in style sheet [v5]

[John Hendrikx]

> This fix introduces immutable sets of `PseudoClass` almost everywhere, as they are rarely modified.  These are re-used by caching them in a new class `ImmutablePseudoClassSetsCache`.
> 
> In order to make this work, `BitSet` had to be cleaned up.  It made assumptions about the collections it is given (which may no longer always be another `BitSet`).  I also added the appropriate null checks to ensure there weren't any other bugs lurking.
> 
> Then there was a severe bug in `toArray` in both the subclasses that implement `BitSet`.
> 
> The bug in `toArray` was incorrect use of the variable `index` which was used for both advancing the pointer in the array to be generated, as well as for the index to the correct `long` in the `BitSet`.  This must have resulted in other hard to reproduce problems when dealing with `Set<PseudoClass>` or `Set<StyleClass>` if directly or indirectly calling `toArray` (which is for example used by `List.of` and `Set.of`) -- I fixed this bug because I need to call `Set.copyOf` which uses `toArray` -- as the same bug was also present in `StyleClassSet`, I fixed it there as well.
> 
> The net result of this change is that there are far fewer `PseudoClassState` objects created; the majority of these are never modified, and the few that are left are where you'd expect to see them modified.
> 
> A test with 160 nested HBoxes which were given the hover state shows a 99.99% reduction in `PseudoClassState` instances and a 70% reduction in heap use (220 MB -> 68 MB), see the linked ticket for more details.
> 
> Although the test case above was extreme, this change should have positive effects for most applications.

John Hendrikx has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 13 commits:

 - Merge branch 'master' of https://git.openjdk.org/jfx into
   feature/immutable-pseudoclassstate
 - Fix another edge case in BitSet equals
   
   When arrays are not the same size, but there are no set bits in the ones
   the other set doesn't have, two bit sets can still be considered equal
 - Take element type into account for BitSet.equals()
 - Base BitSet on AbstractSet to inherit correct equals/hashCode/toArray
   
   - Removed faulty toArray implementations in PseudoClassState and
   StyleClassSet
   - Added test that verifies equals/hashCode for PseudoClassState respect
   Set contract now
   - Made getBits package private so it can't be inherited
 - Remove unused code
 - Ensure Match doesn't allow modification
 - Simplify ImmutablePseudoClassSetsCache and avoid an unnecessary copy
 - Remove double call to getIndex
 - Fix null test case
 - Fix null addAll call
 - ... and 3 more: https://git.openjdk.org/jfx/compare/559ad113...bcf33aa0

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Changes: https://git.openjdk.org/jfx/pull/1076/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1076&range=04
  Stats: 496 lines in 12 files changed: 279 ins; 146 del; 71 mod
  Patch: https://git.openjdk.org/jfx/pull/1076.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1076/head:pull/1076

PR: https://git.openjdk.org/jfx/pull/1076