RFR: JDK-8306990: The guarantees given by Region's floor and ceiling functions should work for larger values [v3]

[John Hendrikx]

> Region has floor and ceiling functions that ensure that calling them twice in a row will yield the same result:
> 
>      ceil(x) = ceil(ceil(x))
> 
> However, due to use of a constant `EPSILON` which is added/subtracted before doing the rounding, this only works for small numbers (in the range of 0-50 approximately).  For larger values and scales, rounding errors can easily occur.  This is visible as artifacts on screen where controls are a pixel wider than they should be.
> 
> The use of the `EPSILON` constant is incorrect, as its value depends on the magnitude of the value in question (as magnitude increases, the fractional precision decreases).
> 
> The Math class offers the function `ulp` that should be used here.  It represents the smallest possible change in value for a given double.
> 
> Extending the existing test case `snappingASnappedValueGivesTheSameValueTest` to use larger magnitude numbers exposes the problems.

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Fix for using constant 1 in HBox/VBox instead of 1 / renderScale

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1118/files
  - new: https://git.openjdk.org/jfx/pull/1118/files/65e1475a..9c9e6391

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1118&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1118&range=01-02

  Stats: 30 lines in 3 files changed: 20 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1118.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1118/head:pull/1118

PR: https://git.openjdk.org/jfx/pull/1118