RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v6]

[Prasanta Sadhukhan]

> Due to transient datatype of scenePeer, it can become null which can result in NPE in scenarios where scene is continuously been reset and set, which warrants a null check, as is done in other places for the same variable.

Prasanta Sadhukhan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - jckeck fix
 - Merge master
 - Locking for all shared resource between Swing and FX thread
 - Locking for all shared resource between Swing and FX thread
 - Modifications as per review comments
 - Check FXEnabled initially
 - Add lock for read/write of scenePeer/stagePeer and added system test
 - 8255248: NullPointerException in JFXPanel due to race condition in HostContainer

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

Changes: https://git.openjdk.org/jfx/pull/1178/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=05
  Stats: 315 lines in 2 files changed: 237 ins; 1 del; 77 mod
  Patch: https://git.openjdk.org/jfx/pull/1178.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1178/head:pull/1178

PR: https://git.openjdk.org/jfx/pull/1178