Set black color for underlying window background to perform correct blending operations in metal with window surface destination
------------- Commit messages: - 8266079: Lanai: AlphaComposite shows differences on Metal compared to OpenGL Changes: https://git.openjdk.java.net/jdk/pull/4206/files Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4206&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8266079 Stats: 244 lines in 2 files changed: 244 ins; 0 del; 0 mod Patch: https://git.openjdk.java.net/jdk/pull/4206.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/4206/head:pull/4206 PR: https://git.openjdk.java.net/jdk/pull/4206
