RFR: 8248301: Lanai - Change MTLStorageMode of MTLSurfaceData texture (render backbuffer) to private

[Ajit Ghaisas]

Issue : https://bugs.openjdk.java.net/browse/JDK-8248301
More details on JBS.

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Commit messages:
 - change texture storage mode

Changes: https://git.openjdk.java.net/lanai/pull/60/files
 Webrev: https://webrevs.openjdk.java.net/lanai/60/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248301
  Stats: 19 lines in 2 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/lanai/pull/60.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/60/head:pull/60

PR: https://git.openjdk.java.net/lanai/pull/60