RFR: 8253301: Lanai: Memory leak in MTLContext code

[Alexey Ushakov]

Moved temp variable to particular MTLContext

-------------

Commit messages:
 - 8253301: Lanai: Memory leak in MTLContext code

Changes: https://git.openjdk.java.net/lanai/pull/102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253301
  Stats: 6 lines in 1 file changed: 1 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/lanai/pull/102.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/102/head:pull/102

PR: https://git.openjdk.java.net/lanai/pull/102