RFR: 8234077: Evaluate ignored unit tests in RenderRootTest

[Ambarish Rapte]

These are 2 corner case test cases for getRenderRoot() method.
1. `emptyDirtyRegion():` When the dirty region rect is empty i.e. (0, 0, -1, -1)
2. `zeroSizeDirtyRegionWithinOpaqueRegion()`: When the dirty region rect is of zero dimensions, for example (20, 20, 0, 0)

- emptyDirtyRegion(): When the dirty region rect is empty i.e. (0, 0, -1, -1)
See `RectBounds.makeEmpty()` and `RectBounds.isEmpty()` for definition of an empty rect. It seems logical to NOT to perform any rendering when dirty region is an empty rect. But current behavior is that when empty dirty region rect is passed to `root.getRenderRoot()`, it returns the root itself in NodePath.
The commit#1 can correct/change this behavior to return an empty NodePath. Commit#1 is only for reference, the change is NGNode.java is a product change which is out of scope of this fix. I shall file a new JBS issue to handle this case.
The test `emptyDirtyRegion1` is modified and another variant(`emptyDirtyRegion2`) of test is added. Additionally a new similar test (`invalidDirtyRegionOutsideOpaqueRegion`) is also included which passes an invalid rect. I think the behavior of this case should also be same as that of empty rect. All these three tests shall be ignored using the new JBS.

- zeroSizeDirtyRegionWithinOpaqueRegion(): When the dirty region rect is of zero dimensions, for example (20, 20, 0, 0)
The dirty rect has 0 width and height but from the methods `RectBounds.makeEmpty()` and `RectBounds.isEmpty()` we can infer that the rect(20, 20, 0, 0) is not an empty rect. (It may be considered as a dirty point). So if we consider this rect as a valid dirty region then the current behavior seems valid. I have added a couple more similar tests.

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Commit messages:
 - revert NGNode.java and ignore tests
 - test correction

Changes: https://git.openjdk.java.net/jfx/pull/463/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=463&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8234077
  Stats: 48 lines in 1 file changed: 43 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/463.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/463/head:pull/463

PR: https://git.openjdk.java.net/jfx/pull/463