RFR: 8271395: Fixing crash at printing

[Florian Kirmaier]

This PR switches the Thread to the QuantumRenderer, in the case the Disposer is called from another Thread - the printing Thread.
I'm open for better solutions on how to fix this Issue.
Initially i thought there is also a Race Condition in the resource pool, but a new one is created for the Printing Thread.

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Commit messages:
 - JDK-8271395 Fixing crash at printing

Changes: https://git.openjdk.java.net/jfx/pull/618/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=618&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271395
  Stats: 15 lines in 2 files changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/618.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/618/head:pull/618

PR: https://git.openjdk.java.net/jfx/pull/618