RFR: JDK-8316590: Rendering artifact after JDK-8311983 [v2]

[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.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8316590: More tests

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1246/files
  - new: https://git.openjdk.org/jfx/pull/1246/files/78586c0e..10f161e6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1246&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1246&range=00-01

  Stats: 39 lines in 1 file changed: 39 ins; 0 del; 0 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