RFR: 8240694: [macos 10.15] JavaFX Media hangs on some video files on Catalina

[Alexander Matveev]

https://bugs.openjdk.java.net/browse/JDK-8240694

- Original fix JDK-8236832 was reverted.
- Timestamp will be queried on event loop thread when spectrum event is received by event loop.
- FIx only enabled for macOS when using OSXPlatform.

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Commit messages:
 - 8240694: [macos 10.15] JavaFX Media hangs on some video files on Catalina

Changes: https://git.openjdk.java.net/jfx/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/jfx/163/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240694
  Stats: 62 lines in 11 files changed: 29 ins; 13 del; 20 mod
  Patch: https://git.openjdk.java.net/jfx/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/163/head:pull/163

PR: https://git.openjdk.java.net/jfx/pull/163