RFR: JDK-8303950: translucent windows flicker

[Jeremy]

I'm confident about the new test case for this ticket, but the resolution is more invasive than I'd like. (It works, though.)

In short:
This introduces a new RenderingHint (in SunHints) to bypass the call in Window to `gg2d.fillRect(0, 0, getWidth(), getHeight());`

I left more detailed notes here about the proposed resolution:
https://github.com/openjdk/jdk/commit/1991fdac5dbf76ddaf73cc78a9f7c38370c9674c

I'm open to suggestions if anyone has a more elegant proposal to prevent the monitor from refreshing too soon?

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Commit messages:
 - 8303950: fixing false negative
 - 8303950: avoid letting Window.paint(..) call g.fillRect(..)
 - 8303950: adding failing test to identify problem
 - Merge pull request #4 from openjdk/master
 - Merge pull request #3 from openjdk/master
 - Merge pull request #2 from openjdk/master
 - Merge pull request #1 from openjdk/master

Changes: https://git.openjdk.org/jdk/pull/12993/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12993&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303950
  Stats: 246 lines in 4 files changed: 241 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12993.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12993/head:pull/12993

PR: https://git.openjdk.org/jdk/pull/12993