RFR: 8309460: JScrollBar leaves behind clutter for non-integer UI scales

[Prasanta Sadhukhan]

Lines  are left behind when moving the scrollbar in the positive direction. but are cleaned up on mouse release. 
Additonally, with right arrow clicks too, the lines are left behind.
Seems like for mouseDragged and scrollByUnit, the dirty region of the scrollbar is not repainted leading to artifacts
which is being done in this fix..

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Commit messages:
 - fix
 - 8309460: JScrollBar leaves behind clutter for non-integer UI scales

Changes: https://git.openjdk.org/jdk/pull/17484/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17484&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309460
  Stats: 82 lines in 2 files changed: 82 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17484.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17484/head:pull/17484

PR: https://git.openjdk.org/jdk/pull/17484