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

[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.

Alexey Ushakov has updated the pull request incrementally with two additional commits since the last revision:

 - 8273355: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1
   
   Override setBackground() for LWWindowPeer to set background color for native window.
 - Revert "8273355: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1"
   
   This reverts commit 47a448c2a0781a79e0175914992af626c2eeb4b4.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7931/files
  - new: https://git.openjdk.java.net/jdk/pull/7931/files/47a448c2..8faccd68

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7931&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7931&range=00-01

  Stats: 36 lines in 5 files changed: 11 ins; 25 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