Integrated: 8251484: Performace drop in FlatBoxAA renderperf test for metal pipeline

[Alexey Ushakov]

Wrapped rendering code in MTLRenderQueue.flushBuffer into autorelease pool to reduce memory consumption.
Removed unnecessary release calls that were done for autoreleased objects.
Removed cached static objects from autorelease pool.

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Commit messages:
 - 8251484: Performace drop in FlatBoxAA renderperf test for metal pipeline

Changes: https://git.openjdk.java.net/lanai/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/lanai/77/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251484
  Stats: 1001 lines in 5 files changed: 275 ins; 261 del; 465 mod
  Patch: https://git.openjdk.java.net/lanai/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/77/head:pull/77

PR: https://git.openjdk.java.net/lanai/pull/77