RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer

[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

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Commit messages:
 - 8255248: NullPointerException in JFXPanel due to race condition in HostContainer

Changes: https://git.openjdk.org/jfx/pull/1968/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8255248
  Stats: 208 lines in 3 files changed: 182 ins; 0 del; 26 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