RFR: 8252386: Lanai: Implement RadialGradientPaint in shader

[Alexey Ushakov]

Implemented RadialGradientPaint, added RenderPerfTest tests

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Commit messages:
 - 8252386: Lanai: Implement RadialGradientPaint in shader

Changes: https://git.openjdk.java.net/lanai/pull/88/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=88&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252386
  Stats: 244 lines in 6 files changed: 223 ins; 16 del; 5 mod
  Patch: https://git.openjdk.java.net/lanai/pull/88.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/88/head:pull/88

PR: https://git.openjdk.java.net/lanai/pull/88