RFR: 8260711:

[Alexey Ushakov]

Lanai: Refactor MTLContext, MTLGraphicsConfig, MTLSurfaceData

Refactoring according to the review comments

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Commit messages:
 - 8260711:

Changes: https://git.openjdk.java.net/lanai/pull/170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=170&range=00
  Stats: 54 lines in 8 files changed: 0 ins; 50 del; 4 mod
  Patch: https://git.openjdk.java.net/lanai/pull/170.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/170/head:pull/170

PR: https://git.openjdk.java.net/lanai/pull/170