RFR: 8273355: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1

[Alexey Ushakov]

Used setOpaque() method to set correct background of platform window
Extracted setting background to the particular method in PlatformWindow. Provided appropriate implementation in CPlatformWindow.

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

Changes: https://git.openjdk.java.net/jdk/pull/7931/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7931&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273355
  Stats: 25 lines in 5 files changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7931.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7931/head:pull/7931

PR: https://git.openjdk.java.net/jdk/pull/7931