RFR: 8088739: [TestBug] RegionBackgroundImageUITest fail with timeout error

[Ambarish Rapte]

Each test in RegionBackgroundImageUITest makes several calls to `robot.getPixelColor()` on App thread. Due to this each test requires more than **60** seconds for execution.

Fix is to save a screen capture of Scene (on App thread) and then read pixel color from the screen capture(not on app thread). This reduces execution time of each test to less than **3** seconds.
Ideally with this fix(commit#1) all the tests should pass. All tests do pass on Windows and Linux but three tests fail on Mac, which used to pass without this change.
- RegionBackgroundImageUITest.unalignedImage
- RegionBackgroundFillUITest.testScenario2
- RegionBackgroundFillUITest.testScenario3

commit#2 solves the above problem. Solution is to fallback to test color again by reading it using `robot.getPixelColor()` on App thread when a test fails.

One test RegionBackgroundImageUITest.unalignedImage_Cover, fails only on Mac platform, before and after this fix.
It is reported as a new issue [JDK-8255679](https://bugs.openjdk.java.net/browse/JDK-8255679)

This is a test fix and affects only the tests that extend from `RegionUITestBase` test class and does not affect other tests.
Verified that `RegionBackgroundImageUITest` and `RegionBackgroundFillUITest` tests pass on all three platforms(except RegionBackgroundImageUITest.unalignedImage_Cover which fails on Mac).

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Commit messages:
 - fix for tests failing on Mac
 - ideal fix, but causes 3 test fail

Changes: https://git.openjdk.java.net/jfx/pull/344/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=344&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8088739
  Stats: 33 lines in 3 files changed: 28 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/344.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/344/head:pull/344

PR: https://git.openjdk.java.net/jfx/pull/344