RFR: 8193445: JavaFX CSS is applied redundantly leading to significant performance degradation

[Ajit Ghaisas]

**Issue :**
https://bugs.openjdk.java.net/browse/JDK-8193445

**Background :**
The CSS performance improvement done in [JDK-8151756](https://bugs.openjdk.java.net/browse/JDK-8151756) had to be backed out due to functional regressions reported in [JDK-8185709](https://bugs.openjdk.java.net/browse/JDK-8185709), [JDK-8183100](https://bugs.openjdk.java.net/browse/JDK-8183100) and [JDK-8168951](https://bugs.openjdk.java.net/browse/JDK-8168951).
Refer to [JDK-8183100](https://bugs.openjdk.java.net/browse/JDK-8183100) for more details on this backout. 

**Description :**
This PR reintroduces the CSS performance improvement fix done in [JDK-8151756](https://bugs.openjdk.java.net/browse/JDK-8151756) while addressing the functional regressions that were reported in [JDK-8185709](https://bugs.openjdk.java.net/browse/JDK-8185709), [JDK-8183100](https://bugs.openjdk.java.net/browse/JDK-8183100) and [JDK-8168951](https://bugs.openjdk.java.net/browse/JDK-8168951).
For ease of review, I have made two separate commits -
1) [Commit 1](https://github.com/openjdk/jfx/pull/34/commits/d964675ebc2a42f2fd6928b773819502683f2334) - Reintroduces the CSS performance improvement fix done in [JDK-8151756](https://bugs.openjdk.java.net/browse/JDK-8151756) - most of the patch applied cleanly.
2) [Commit 2 ](https://github.com/openjdk/jfx/pull/34/commits/12ea8220a730ff8d98d520ce870691d54f0de00f)- fixes the functional regressions keeping performance improvement intact + adds a system test

**Root Cause :**
CSS performance improvement fix proposed in [JDK-8151756 ](https://bugs.openjdk.java.net/browse/JDK-8151756)correctly avoids the redundant CSS reapplication to children of a Parent. 
What was missed earlier in [JDK-8151756 ](https://bugs.openjdk.java.net/browse/JDK-8151756) fix : "CSS reapplication to the Parent itself”. 
This missing piece was the root cause of all functional regressions reported against [JDK-8151756](https://bugs.openjdk.java.net/browse/JDK-8151756)

**Fix :**
Fixed the identified root cause. See commit 2.

**Testing :**
1. All passing unit tests continue to pass
2. New system test (based on [JDK-8209830](https://bugs.openjdk.java.net/browse/JDK-8209830)) added in this PR - fails before this fix and passes after the fix
3. System test JDK8183100Test continues to pass
4. All test cases attached to regressions [JDK-8185709](https://bugs.openjdk.java.net/browse/JDK-8185709), [JDK-8183100](https://bugs.openjdk.java.net/browse/JDK-8183100) and [JDK-8168951](https://bugs.openjdk.java.net/browse/JDK-8168951) pass with this fix

In addition, testing by community with specific CSS performance / functionality will be helpful.

----------------

Commits:
 - 12ea8220: Fix for functional regressions of JDK-8151756 + add a sytem test
 - d964675e: Reintroduce JDK-8151756 CSS performance fix

Changes: https://git.openjdk.java.net/jfx/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/jfx/34/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8193445
  Stats: 121 lines in 5 files changed: 104 ins; 0 del; 17 mod
  Patch: https://git.openjdk.java.net/jfx/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/34/head:pull/34

PR: https://git.openjdk.java.net/jfx/pull/34