RFR: 8377426: Adjust scene background to color scheme [v6]

[Michael Strauß]

> When a non-opaque scene fill color is used with a stage style other than `StageStyle.TRANSPARENT`, the actual fill color is always white. This doesn't work well when the scene uses a dark color scheme. A practical solution is to allow non-opaque scene fill colors, and blend them on top of a white or black background (depending on color scheme) to derive an opaque color that adapts intuitively to the color scheme.
> 
> To test this, simply create a scene that uses a non-opaque fill color and observe the scene background when the color scheme is changed.
> 
> This PR includes a system test, run it with:
> 
> ./gradlew -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test --tests test.robot.javafx.scene.SceneFillTest.testSceneFill

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  check FX thread

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Changes:
  - all: https://git.openjdk.org/jfx/pull/2068/files
  - new: https://git.openjdk.org/jfx/pull/2068/files/da450faf..2f66dd9b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2068&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2068&range=04-05

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2068.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2068/head:pull/2068

PR: https://git.openjdk.org/jfx/pull/2068