RFR: 8259853: Lanai: nonAA Gradient painting is not precise for VI

[Alexey Ushakov]

Added offset for correct rendering

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Commit messages:
 - 8259853: Lanai: nonAA Gradient painting is not precise for VI

Changes: https://git.openjdk.java.net/lanai/pull/155/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=155&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259853
  Stats: 29 lines in 1 file changed: 23 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/lanai/pull/155.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/155/head:pull/155

PR: https://git.openjdk.java.net/lanai/pull/155