RFR: 8304831: TextFlow.hitTest.insertionIndex incorrect with surrogate pairs

[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.

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Commit messages:
 - Fix insertion index calculation issue in TextFlow

Changes: https://git.openjdk.org/jfx/pull/1091/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1091&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304831
  Stats: 182 lines in 2 files changed: 181 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