Integrated: 8242925 J2DDemo - Anti-Aliasing with Metal differs from OGL

[Alexey Ushakov]

Corrected alpha blending in shader

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Commit messages:
 - 8242925 J2DDemo - Anti-Aliased rendering with Metal differs as compared to OpenGL

Changes: https://git.openjdk.java.net/lanai/pull/139/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=139&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242925
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/lanai/pull/139.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/139/head:pull/139

PR: https://git.openjdk.java.net/lanai/pull/139