RFR: 8261712: Lanai: Crash in MTLClip when Metal API Validation enabled

[Alexey Ushakov]

Do not use stencil in stencil mask generation

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Commit messages:
 - 8261712: Lanai: Crash in MTLClip when Metal API Validation enabled

Changes: https://git.openjdk.java.net/lanai/pull/189/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=189&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261712
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/lanai/pull/189.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/189/head:pull/189

PR: https://git.openjdk.java.net/lanai/pull/189