RFR: JDK-8240996: Lanai: rendering artifacts with external GPU

[Alexey Ushakov]

Added MTLTextureUsageUnknown usage to texture surface creation code

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

Commit messages:
 - JDK-8240996: Lanai: rendering artifacts with external GPU

Changes: https://git.openjdk.java.net/lanai/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/lanai/8/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8240996
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/lanai/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/8/head:pull/8

PR: https://git.openjdk.java.net/lanai/pull/8