RFR: [WIP] 8088739: [TestBug] RegionBackgroundImageUITest fail with timeout error [v2]

[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).

Ambarish Rapte has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge
 - fix for tests failing on Mac
 - ideal fix, but causes 3 test fail

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/344/files
  - new: https://git.openjdk.java.net/jfx/pull/344/files/91791f3a..f270a0bb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=344&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=344&range=00-01

  Stats: 376323 lines in 7058 files changed: 185953 ins; 125920 del; 64450 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