RFR: 8273355: Lanai: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1 [v2]

[Alexey Ushakov]

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

Alexey Ushakov has updated the pull request incrementally with one additional commit since the last revision:

  8273355: Lanai: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1
  
  Extracted setting background to the particular method in PlatformWindow. Provided appropriate implementation in CPlatformWindow.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5373/files
  - new: https://git.openjdk.java.net/jdk/pull/5373/files/752c2133..5027f885

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5373&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5373&range=00-01

  Stats: 24 lines in 5 files changed: 22 ins; 0 del; 2 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