RFR: 8298728: Cells in VirtualFlow jump after resizing

[Johan Vos]

When recalculating sizes, we often don't want the current index and/or offset to change.

Allow to fix the index/offset when doing recalculations.

Fix JDK-8298728

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

Commit messages:
 - When recalculating sizes, we often don't want the current index and/or offset to change.

Changes: https://git.openjdk.org/jfx/pull/974/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=974&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298728
  Stats: 113 lines in 4 files changed: 96 ins; 1 del; 16 mod
  Patch: https://git.openjdk.org/jfx/pull/974.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/974/head:pull/974

PR: https://git.openjdk.org/jfx/pull/974