RFR: 8290344: Start/stop displaysync affects performance in metal rendering pipeline

[Alexey Ushakov]

Reuse displaysync thread for subsequent updates

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Commit messages:
 - 8290344: Start/stop displaysync affects performance in metal rendering pipeline

Changes: https://git.openjdk.org/jdk/pull/9512/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290344
  Stats: 22 lines in 2 files changed: 15 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/9512.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9512/head:pull/9512

PR: https://git.openjdk.org/jdk/pull/9512