RFR: 8201567: QuantumRenderer modifies buffer in use by JavaFX Application Thread

[John Neffenger]

Fixes [JDK-8201567](https://bugs.openjdk.java.net/browse/JDK-8201567).

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Commit messages:
 - 8201567: QuantumRenderer modifies buffer in use by JavaFX Application Thread

Changes: https://git.openjdk.java.net/jfx/pull/255/files
 Webrev: https://webrevs.openjdk.java.net/jfx/255/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8201567
  Stats: 35 lines in 5 files changed: 29 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/255.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/255/head:pull/255

PR: https://git.openjdk.java.net/jfx/pull/255