RFR: 8251167: Drawing polyline twice in XOR mode leaves out some traces on screen (only with uiScale=1.0)

[Ajit Ghaisas]

Issue : https://bugs.openjdk.java.net/browse/JDK-8251167

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Commit messages:
 - XOR mode polyline fix

Changes: https://git.openjdk.java.net/lanai/pull/74/files
 Webrev: https://webrevs.openjdk.java.net/lanai/74/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251167
  Stats: 27 lines in 1 file changed: 20 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/lanai/pull/74.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/74/head:pull/74

PR: https://git.openjdk.java.net/lanai/pull/74