<AWT Dev> RFR: 8273355: Lanai: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1

[Alexey Ushakov]

Used setOpaque() method to set correct background of platform window

-------------

Commit messages:
 - 8273355: Lanai: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1

Changes: https://git.openjdk.java.net/jdk/pull/5373/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5373&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273355
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5373.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5373/head:pull/5373

PR: https://git.openjdk.java.net/jdk/pull/5373