RFR: JDK-8305248: TableView not rendered correctly after column is made visible if fixed cell size is set

[Marius Hanl]

The determined `prefWidth` of a `TableCell` could be `0.0` when a `fixedCellSize` is set.
This happened because the `TableCell` may not have a skin since it was never added to the scene graph yet.

The fix is to make sure we get the `prefWidth` after the `TableCell` was added to the scene graph.
That is also the reason why the problem only happened the first time and never again after (skin is then already created).

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Commit messages:
 - JDK-8305248: TableView not rendered correctly after column is made visible if fixed cell size is set

Changes: https://git.openjdk.org/jfx/pull/1077/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1077&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305248
  Stats: 52 lines in 2 files changed: 48 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1077.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1077/head:pull/1077

PR: https://git.openjdk.org/jfx/pull/1077