RFR: 8305352: updateIconImages may lead to deadlock after JDK-8276849

[Dmitry Cherepanov]

Please review this PR which suggests to skip updateIconImages() for not yet visible windows. The displayChanged notification can be sent to a window that is in the process of initiailization and calling updateIconImages() may lead to the deadlock.

Testing: the deadlock no longer happens with this patch. The reg test for JDK-8276849 works (window icon is updated after changing DPI settings).

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Commit messages:
 - skip updateIconImages for invisible windows

Changes: https://git.openjdk.org/jdk/pull/13263/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13263&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305352
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13263.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13263/head:pull/13263

PR: https://git.openjdk.org/jdk/pull/13263