RFR: 8262882: Lanai: NetBeans crashes often when switching between dual and single screen

[Ajit Ghaisas]

Corrected MTLContext.dealloc() method to set released members to nil.
Setting texturePool member to nil causes double release as texturePool content gets released in MTLCommandBufferWrapper.onComplete() method as well.

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Commit messages:
 - fix MTLContext dispose

Changes: https://git.openjdk.java.net/lanai/pull/211/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=211&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262882
  Stats: 59 lines in 3 files changed: 46 ins; 4 del; 9 mod
  Patch: https://git.openjdk.java.net/lanai/pull/211.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/211/head:pull/211

PR: https://git.openjdk.java.net/lanai/pull/211