RFR: 8257719: JFXPanel scene fails to render correctly on HiDPI after fix for JDK-8199592

[Jose Pereda]

This PR fixes a regression introduced in [JDK-8199592](https://bugs.openjdk.java.net/browse/JDK-8199592), where scene initialization was moved after updating output scales. However, in case of embedded windows (used by `JFXPanel` or `FXCanvas`), the scene has to be initialized before.

A test has been included. Before applying the fix, the JFXPanel is rendered with a garbled image, and after the fix, the rendering is correct.

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Commit messages:
 - Initialize scene of embedded windows before updating output scales, including test

Changes: https://git.openjdk.java.net/jfx/pull/364/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=364&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257719
  Stats: 180 lines in 2 files changed: 179 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/364.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/364/head:pull/364

PR: https://git.openjdk.java.net/jfx/pull/364