RFR: 8309470: Potential performance improvements in VirtualFlow

[Marius Hanl]

This PR does two small improvements to `VirtualFlow`.
Until now, the `VirtualFlow` sometimes called the `computeHeight` or `computeWidth` methods, although a fixed cell size is set and we therefore don't need to call those method (and never should do, as we may don't get the expected result and mix computed sizes with the fixed cell size).

Added tests that fail before and pass now. They check that the `computeHeight` or `computeWidth` (non vertical flow) are never called when a fixed cell size is set.

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Commit messages:
 - JDK-8309470: Potential performance improvements in VirtualFlow

Changes: https://git.openjdk.org/jfx/pull/1150/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1150&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309470
  Stats: 126 lines in 2 files changed: 101 ins; 13 del; 12 mod
  Patch: https://git.openjdk.org/jfx/pull/1150.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1150/head:pull/1150

PR: https://git.openjdk.org/jfx/pull/1150