RFR: 8371069: RichTextArea: caret with inline node

[Andy Goryachev]

Original bug referred to RichTextArea, however the root cause of the issue is the incorrect computation of the caret shape around an embedded `Node` in the `TextFlow`.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jfx/pull/1974/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1974&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371069
  Stats: 17 lines in 2 files changed: 16 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1974.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1974/head:pull/1974

PR: https://git.openjdk.org/jfx/pull/1974