RFR: 8258955: XBuffer.slice(int, int) fails to adjust index according to primitive size

[Chris Hegarty]

Scale the slice start index per carrier width, for views of direct byte buffers. This never worked correctly since being added in Java 13.

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Commit messages:
 - Fix direct buffer slice

Changes: https://git.openjdk.java.net/jdk/pull/1906/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1906&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258955
  Stats: 35 lines in 2 files changed: 31 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1906.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1906/head:pull/1906

PR: https://git.openjdk.java.net/jdk/pull/1906