RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v11]

[Prasanta Sadhukhan]

> NPE is seen while accessing transient "scenePeer" variable between reads..
> Fix is made to store it in a temp variable rather than reading it twice since the value can change between successive reads in many places it is accessed.
> Also some debug logs added to be enabled via `jfxpanel.debug` property

Prasanta Sadhukhan has updated the pull request incrementally with two additional commits since the last revision:

 - Rename var w.r.t the accessing thread for easy perusal
 - comment fix

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1968/files
  - new: https://git.openjdk.org/jfx/pull/1968/files/e0c5a363..2e8d0997

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=10
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=09-10

  Stats: 41 lines in 2 files changed: 0 ins; 0 del; 41 mod
  Patch: https://git.openjdk.org/jfx/pull/1968.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1968/head:pull/1968

PR: https://git.openjdk.org/jfx/pull/1968