RFR: 8282386: JavaFX media stubs rely on libav.org

[Johan Vos]

Retrieve libav sources from github.
Fix JDK-8282386

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Commit messages:
 - Retrieve libav sources from github.

Changes: https://git.openjdk.org/jfx/pull/989/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=989&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8282386
  Stats: 19 lines in 2 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/989.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/989/head:pull/989

PR: https://git.openjdk.org/jfx/pull/989