RFR: 8311527: Region.snapInnerSpace*()

[Andy Goryachev]

Introduces Region.snapInnerSpaceX/Y() methods for dealing with inner space (using Math.floor), see for instance [JDK-8299753](https://bugs.openjdk.org/browse/JDK-8299753), using existing methods Region.snapPortionX/Y().

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Commit messages:
 - tests
 - Merge remote-tracking branch 'origin/master' into ag.8311527.snap.inner
 - javadoc
 - snap portion
 - cleanup
 - 8311527: snap inner space

Changes: https://git.openjdk.org/jfx/pull/1190/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1190&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311527
  Stats: 88 lines in 2 files changed: 87 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1190.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1190/head:pull/1190

PR: https://git.openjdk.org/jfx/pull/1190