RFR: 8321722: Tab header flickering when dragging slowly other tabs and reordering uncompleted

[Jose Pereda]

This PR prevents changes in direction when dragging a tab header over a TabPane control, if delta value is zero, which can happen with slow drag events, in order to prevent starting and stopping too frequently the animation of the target tab header in opposite directions.

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Commit messages:
 - Prevent changes in direction when dragging if delta value is zero

Changes: https://git.openjdk.org/jfx/pull/1304/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1304&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321722
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1304.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1304/head:pull/1304

PR: https://git.openjdk.org/jfx/pull/1304