RFR: 8201567: QuantumRenderer modifies buffer in use by JavaFX Application Thread [v2]

[John Neffenger]

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

John Neffenger has updated the pull request incrementally with one additional commit since the last revision:

  Remove assert statements

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/255/files
  - new: https://git.openjdk.java.net/jfx/pull/255/files/47ad942a..d408506a

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/255/webrev.01
 - incr: https://webrevs.openjdk.java.net/jfx/255/webrev.00-01

  Stats: 21 lines in 3 files changed: 5 ins; 14 del; 2 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