RFR: 8178297: TableView scrolls slightly when adding new elements

[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.

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

Commit messages:
 - 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=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8178297
  Stats: 25 lines in 2 files changed: 23 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