RFR: 8333919: dragViewOffsetX/dragViewOffsetY are ignored for the dragView image

[Lukasz Kostyra]

When fixing [JDK-8233955](https://bugs.openjdk.org/browse/JDK-8233955) offset calculation was left out, which made Dragboard offset API not work on macOS.

This change fixes this mistake. Now drag image should be properly offset.

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Commit messages:
 - GlassViewDelegate: Restore offset calculation on startDrag

Changes: https://git.openjdk.org/jfx/pull/1532/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1532&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333919
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1532.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1532/head:pull/1532

PR: https://git.openjdk.org/jfx/pull/1532