[jfx11u] RFR: 8259356: MediaPlayer's seek freezes video

[Kevin Rushforth]

Clean backport to jfx11u. Tested on all three platforms, along with my other in-progress backports, all of which are aggregated in my [`kevinrushforth:test-kcr-11.0.12`](https://github.com/kevinrushforth/jfx11u/commits/test-kcr-11.0.12) branch.

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Commit messages:
 - 8259356: MediaPlayer's seek freezes video

Changes: https://git.openjdk.java.net/jfx11u/pull/10/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=10&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259356
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/10.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/10/head:pull/10

PR: https://git.openjdk.java.net/jfx11u/pull/10