RFR: 8187759: Background not refreshed when painting over a transparent JFrame

[Tejesh R]

This is happening in linux where `BuffereStrategyPaintManager` is used to paint to offscreen. Here `bsg` bufferStrategy SunGraphics2D is used to paint to offscreen where the background is not refreshed, which does only clipping and then paints to offscreen. In order to handle the screen updated/clear the buffer, `setBackground` to component background color and `clearRect` against the clip area is used which solves the issue without causing any regression. 
CI is green for the fix and manual test is provided.

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Commit messages:
 - Linux tag added
 - fix + manual test

Changes: https://git.openjdk.org/jdk/pull/17081/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8187759
  Stats: 139 lines in 2 files changed: 139 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17081.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17081/head:pull/17081

PR: https://git.openjdk.org/jdk/pull/17081