RFR: 8304831: TextFlow.hitTest.insertionIndex incorrect with surrogate pairs [v6]

[Karthik P K]

> Since surrogate pairs are internally considered as 2 characters and text field is null in `HitInfo` when `getInsertionIndex` is invoked from `TextFlow`, wrong insertion index was returned.
> 
> Updated code to calculate insertion index in `getHitInfo` method of `PrismTextLayout` class when `hitTest` of trailing side of surrogate pair is requested. Since text runs are processed in this method already, calculating the insertion index in this method looks better than calculating in `getInsertionIndex` of `HitInfo` method.
> The latter approach also requires the text to be sent to `HitInfo` as parameter from the `hitTest` method of `TextFlow`. If the number of `Text` nodes in `TextFlow` are very large, processing all the `Text` nodes on each `hitTest` method invocation might cause performance issue. Hence implemented first approach.
> 
> Added system test to validate the fix.

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Add new test. Optimizations to make the tests robust

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1091/files
  - new: https://git.openjdk.org/jfx/pull/1091/files/1eb11749..a43bbdb2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1091&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1091&range=04-05

  Stats: 108 lines in 1 file changed: 70 ins; 24 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1091.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1091/head:pull/1091

PR: https://git.openjdk.org/jfx/pull/1091