RFR: 8365576: Temporarily make Metal the default JavaFX rendering pipeline for macOS

[Ambarish Rapte]

On macOS, temporarily set the default rendering pipeline to Metal, for the next ~7 weeks (until Oct 2).
This is done to get more testing of the Metal pipeline.
If you need to use the ES2 pipeline, add **-Dprism.order=es2** option to the Java command.

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Commit messages:
 - 8365576: Temporarily make Metal the default JavaFX rendering pipeline for macOS

Changes: https://git.openjdk.org/jfx/pull/1876/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1876&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365576
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1876.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1876/head:pull/1876

PR: https://git.openjdk.org/jfx/pull/1876