RFR: 8187759: Background not refreshed when painting over a transparent JFrame [v2]

[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.

Tejesh R has updated the pull request incrementally with one additional commit since the last revision:

  Review fix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17081/files
  - new: https://git.openjdk.org/jdk/pull/17081/files/925b1b06..c72d9295

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17081&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17081&range=00-01

  Stats: 7 lines in 1 file changed: 5 ins; 1 del; 1 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