RFR: 8274137: TableView scrollbar/header misaligned when reloading data

[Robert Lichtenberger]

This PR fixes JDK-8274137 by removing the optimization from updateHbar() that will no-op the method in case the VirtualFlow is invisible or currently has no scene.
Since changes to the hBar's value can happen even if the VirtualFlow is not currently visible, the synchronisation between hBar and clipX must happen all the time.

A test agains VirtualFlow has been added that will fail before the change and pass afterwards.

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Commit messages:
 - 8274137: TableView scrollbar/header misaligned when reloading data.
 - 8274137: TableView scrollbar/header misaligned when reloading data.
 - 8274137: TableView scrollbar/header misaligned when reloading data

Changes: https://git.openjdk.java.net/jfx/pull/629/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=629&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274137
  Stats: 33 lines in 3 files changed: 32 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/629.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/629/head:pull/629

PR: https://git.openjdk.java.net/jfx/pull/629