[OpenJDK 2D-Dev] RFR: 8266079: Lanai: AlphaComposite shows differences on Metal compared to OpenGL

[Alexey Ushakov]

Keep MTLLayer opacity in sync with window content view
Keep layer translucent for translucent windows

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Commit messages:
 - 8266079: Lanai: AlphaComposite shows differences on Metal compared to OpenGL

Changes: https://git.openjdk.java.net/jdk/pull/4946/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4946&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266079
  Stats: 270 lines in 7 files changed: 266 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4946.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4946/head:pull/4946

PR: https://git.openjdk.java.net/jdk/pull/4946