RFR: JDK-8263389 IGV: Zooming changes the point that is currently centered

[jtfuller111]

Fixing the zoom issue on IGV. I tested this on Windows and Linux and zooming now works as expected.

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Commit messages:
 - IGV: Zooming now preserves the point currently centered

Changes: https://git.openjdk.java.net/jdk/pull/4291/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4291&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263389
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4291.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4291/head:pull/4291

PR: https://git.openjdk.java.net/jdk/pull/4291