RFR: 8304831: TextFlow.hitTest.insertionIndex incorrect with surrogate pairs [v8]

[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 with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Merge master to resolve merge conflict
 - Stabilizing the system tests
 - Add new test. Optimizations to make the tests robust
 - Change insertion index initialization approach. Add additional test
 - Initialize insertion index in PrismTextLayout
 - Address code review
 - Fix insertion index calculation issue with emojis. Add additional test cases
 - Fix insertion index calculation issue in TextFlow

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Changes: https://git.openjdk.org/jfx/pull/1091/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1091&range=07
  Stats: 408 lines in 3 files changed: 407 ins; 0 del; 1 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