RFR: 8242530: [macos] Some audio files miss spectrum data when another audio file plays first

[Alexander Matveev]

https://bugs.openjdk.java.net/browse/JDK-8242530

- GstElementClass which is base class for all elements has same instance between all spectrum elements (not sure if it is
  same for all elements) and thus post_message was sending events to AVFoundation callback from GStreamer platform. This
  issue is reproducible if we load OSXPlatfrom first and then play media files using GStreamer. Fixed by introducing
  separate callback to send messages to AVFoundation.

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Commit messages:
 - 8242530: [macos] Some audio files miss spectrum data when another audio file plays first

Changes: https://git.openjdk.java.net/jfx/pull/184/files
 Webrev: https://webrevs.openjdk.java.net/jfx/184/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242530
  Stats: 23 lines in 3 files changed: 19 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/184.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/184/head:pull/184

PR: https://git.openjdk.java.net/jfx/pull/184