RFR: 8257566: Lanai: System runs out of application memory while running the Unmanaged_BufferredImage_draw_NearestNeighbor test multiple times

[Ajit Ghaisas]

This PR fixes a memory leak introduced recently.
It was introduced by the fix of JDK-8238285 and missed to be corrected in the fix of JDK-8257413.

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Commit messages:
 - make temp buffers autorelease

Changes: https://git.openjdk.java.net/lanai/pull/137/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=137&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257566
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/lanai/pull/137.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/137/head:pull/137

PR: https://git.openjdk.java.net/lanai/pull/137