RFR: JDK-8303950: [macos]Translucent Windows Flicker on Repaint

[Jeremy]

# Problem Summary

For non-opaque windows, Window#paint calls `gg.fillRect(0, 0, getWidth(), getHeight())` before `super.paint(g)`.

This can cause flickering on Mac, and the flickering seems to have gotten much worse in recent JVMs. (See movie attachments to original ticket.)

# Discussion

This is my 2nd PR for this ticket. The original is [here](https://github.com/openjdk/jdk/pull/12993); that proposed change was IMO more invasive/scarier. It was auto-closed after 8 weeks of inactivity, and I'd rather offer this PR instead.

In that previous discussion Alan Snyder framed the core problem as a "lack of synchronization" (see [comment here](https://github.com/openjdk/jdk/pull/12993#issuecomment-1467528061)). If the monitor refreshes/flushes after we've called `fillRect` *but before we finish `super.paint`*: it makes sense that we'd see a flicker.

I agree with Alan, but I think the problem can also be framed as a mixing-Swing-with-AWT issue. (Which IMO will involve an easier fix.)

This PR is a low-risk change (relative to the previous PR) that intercepts calls to repaint a Window that is also RootPaneContainer. Now we'll redirect those calls to paint the JRootPane instead. This means we'll exclusively paint within Swing's/RepaintManager's double-buffered architecture, so we bypass the risky call to `fillRect` on the screen's Graphics2D. (And this change occurs within RepaintManager, so we're clearly already in Swing's architecture.)

So with this change: we paint everything to the double-buffer, and the *only time* we paint to the Window's Graphics2D is when have set up a AlphaComposite.Src and replace its contents with our buffer's contents.

# Tests

This PR includes a new test for 8303950 itself. This is pretty self-explanatory: we repaint a trivial animation for a few seconds and use the Robot to see if a pixel is the expected color.

This PR also includes a test called `bug8303950_legacyWindowPaintBehavior` that creates a grid of 4 windows with varying opacity/backgrounds:

<img width="805" alt="image" src="https://github.com/openjdk/jdk/assets/7669569/497c21a0-ed18-413f-a5c7-3ea146644fd6">

I was surprised by how these windows rendered, but I don't think that's worth debating here. This test simply makes sure that we preserve this preexisting behavior. The broad "rules" appear to be:
1. If a JComponent identifies as opaque (see `JComponent.isOpaque`) then the JComponent's background is used. (In this case: that's the opaque green top two windows.) This is probably coming from `ComponentUI.update`
2. If a JRootPane has a translucent background, that color "wins". It doesn't composite on top of a window's translucent background: it simply *replaces* it. (See the blue-ish bottom-right window. Note it's blue -- not purple.) This is probably because the RepaintManager uses `AlphaComposite.Src` in a couple of places to replace its destination Graphics.
3. If a JRootPane has a null background color, then the Window's background color paints as expected. (See the red-ish bottom-left window.)

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Commit messages:
 - 8303950: working on occasional unit failures
 - 8303950: avoid System.exit(1)
 - 8303950: adding opaque white window under test windows
 - 8303950: this resolves bug8303950 by adding special RootPaneContainer behavior
 - 8303950: adding unit test for legacy window behavior
 - 8303950: fixing false negative
 - 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
 - ... and 1 more: https://git.openjdk.org/jdk/compare/98a7a60f...37834391

Changes: https://git.openjdk.org/jdk/pull/14363/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14363&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303950
  Stats: 409 lines in 3 files changed: 407 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14363.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14363/head:pull/14363

PR: https://git.openjdk.org/jdk/pull/14363