Integrated: 8262928: Commit commandbuffer and gracefully override MTLContext

[Jayathirth D V]

This is a follow up code verification task after https://bugs.openjdk.java.net/browse/JDK-8262750.
If we are overriding an active MTLContext we need to commit the GPU queue to maintain proper state.

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Commit messages:
 - 8262928: Commit commandbuffer and gracefully override MTLContext

Changes: https://git.openjdk.java.net/lanai/pull/208/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=208&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262928
  Stats: 8 lines in 1 file changed: 7 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/lanai/pull/208.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/208/head:pull/208

PR: https://git.openjdk.java.net/lanai/pull/208