Integrated: 8258257 Lanai: optimize stencil texture clear code

[Alexey Ushakov]

8258257 Lanai: optimize stencil texture clear code

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

Commit messages:
 - 8258257 Lanai: optimize stencil texture clear code

Changes: https://git.openjdk.java.net/lanai/pull/144/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=144&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258257
  Stats: 32 lines in 2 files changed: 14 ins; 15 del; 3 mod
  Patch: https://git.openjdk.java.net/lanai/pull/144.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/144/head:pull/144

PR: https://git.openjdk.java.net/lanai/pull/144