RFR: 8252895: Black background in SwingSet2 in Nimbus LAF

[Alexey Ushakov]

Used color alpha in gradient shaders

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Commit messages:
 - 8252895: Black background in SwingSet2 in Nimbus LAF

Changes: https://git.openjdk.java.net/lanai/pull/92/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=92&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252895
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/lanai/pull/92.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/92/head:pull/92

PR: https://git.openjdk.java.net/lanai/pull/92