RFR: 8276313: ScrollPane scroll delta incorrectly depends on content height

[Michael Strauß]

This PR fixes an issue where the scroll delta of ScrollPane incorrectly depends on the size of its content.
This leads to extremely slow scrolling when the content is only slightly larger than the ScrollPane.

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

Commit messages:
 - Fixed scrolling speed for ScrollPaneSkin
 - failing test

Changes: https://git.openjdk.java.net/jfx/pull/660/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=660&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276313
  Stats: 52 lines in 2 files changed: 49 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/660.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/660/head:pull/660

PR: https://git.openjdk.java.net/jfx/pull/660