Integrated: 8252385: Lanai: Implement LinearGradient paint in shader

[Alexey Ushakov]

Implemented multi-stop linear gradient paint in shader. Added RenderPerf tests.

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Commit messages:
 - 8252385: Lanai: Implement LinearGradient paint in shader

Changes: https://git.openjdk.java.net/lanai/pull/85/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=85&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252385
  Stats: 226 lines in 9 files changed: 199 ins; 0 del; 27 mod
  Patch: https://git.openjdk.java.net/lanai/pull/85.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/85/head:pull/85

PR: https://git.openjdk.java.net/lanai/pull/85