[jfx11u] RFR: 8160597: IllegalArgumentException when we initiate drag on Image

[Johan Vos]

Clean backport for 8160597: IllegalArgumentException when we initiate drag on Image

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Commit messages:
 - 8160597: IllegalArgumentException when we initiate drag on Image

Changes: https://git.openjdk.java.net/jfx11u/pull/63/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=63&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8160597
  Stats: 157 lines in 5 files changed: 156 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/63.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/63/head:pull/63

PR: https://git.openjdk.java.net/jfx11u/pull/63