RFR: 8267160: Monocle mouse never get ENTERED state

[Johan Vos]

allow to pass a fallback window in case the existing one is null (or can't be computed).

Fix for JDK-8267160

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Commit messages:
 - allow to pass a fallback window in case the existing one is null (or can't be computed).

Changes: https://git.openjdk.java.net/jfx/pull/502/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=502&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267160
  Stats: 13 lines in 2 files changed: 7 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/502.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/502/head:pull/502

PR: https://git.openjdk.java.net/jfx/pull/502