RFR: 8178297: TableView scrolls slightly when adding new elements [v2]

[Jose Pereda]

> This PR fixes an issue that happens when adding new items to a `TableView` or any other control that uses
> `VirtualFlow`, if the scroll position is not 0: the virtual flow is slightly shifted down.
> For instance, let's say that a cell has a layoutY of -10.0. After adding a new item to the table, during the layout
> pass the new value is initially modified to -9.999999999999996 (`VirtualFlow::adjustByPixelAmount`) , and then, after
> calling `VirtualFlow::positionCell` (that uses `snapSizeY(position)`) the layoutY is modified to -9.5, causing an
> undesired positive shift of 0.5 pixels.  `Region` has different snap methods:  `snapSizeX/Y` are used to snap node
> dimension values, like width or height, by ceiling to the nearest pixel (which explains the -9.5 value), while
> `snapSpaceX/Y` are used to snap position values like insets, by rounding to the nearest pixel (this will give the
> expected -10.0).   Therefore, this PR modifies `VirtualFlow::positionCell` to use the `snapSpaceX/Y` methods, since
> these are being used to set the layout of the cell and not its size.  Test: A unity test has been included. It
> simulates the case of adding new items to the virtual flow after an initial scroll. It currently fails after adding a
> few items, and it passes with the changes of this PR.

Jose Pereda has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains three commits:

 - Merge master
 - Add test to verify the cell position remains constant when new cells are added to the virtualFlow.
 - Use correct snap method to set cell layout

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

Changes: https://git.openjdk.java.net/jfx/pull/297/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=297&range=01
  Stats: 24 lines in 2 files changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/297.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/297/head:pull/297

PR: https://git.openjdk.java.net/jfx/pull/297