RFR: 8287912: GTK L&F : Background of tree icons are red

[Abhishek Kumar]

The background of tree icons are not red in GTK LAF when setOpaque is set to false for tree component.
It has been observed that while painting tree cell background in GTK LAF, a rectangular area is also painted with background color (white). 

Proposed solution is to check the opacity of tree component before drawing tree cell background. If the opacity is set to "false" then the background shouldn't be painted.

An automated test case has been added and checked in CI, link is added in JBS.

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Commit messages:
 - Whitespace error fixed
 - Fix for tree icon background in GTK LAF

Changes: https://git.openjdk.org/jdk/pull/10112/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10112&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287912
  Stats: 116 lines in 2 files changed: 114 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10112.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10112/head:pull/10112

PR: https://git.openjdk.org/jdk/pull/10112