RFR: JDK-8316590: Rendering artifact after JDK-8311983

[Marius Hanl]

Fixes the regression by basically reverting one change introduced in https://bugs.openjdk.org/browse/JDK-8311983.
The problem is that it is actually required to get the size from a cell with the index -1, which technically does not exist (the accumCell is used then).

Furthermore, unlike the name suggests, the call to `addLeadingCells()` is always needed, even if there are none.
This is because the method does much more than what you would think first. 

In future, it would probably be good to revisit this code. Also performance wise, since this looks like something that can be optimized. But that is another story, not related to this fix.

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Commit messages:
 - JDK-8316590: Rendering artifact after JDK-8311983

Changes: https://git.openjdk.org/jfx/pull/1246/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1246&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316590
  Stats: 28 lines in 2 files changed: 23 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1246.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1246/head:pull/1246

PR: https://git.openjdk.org/jfx/pull/1246