Integrated: 8252795: Lanai: Refactor native implementation of MTLPaint

[Alexey Ushakov]

Created base class MTLPaint responsible only for blit rendering
Added MTLColorPaint, MTLGradPaint, ... classes representing particular paint modes

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Commit messages:
 - 8252795: Lanai: Refactor native implementation of MTLPaint

Changes: https://git.openjdk.java.net/lanai/pull/98/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=98&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252795
  Stats: 1153 lines in 6 files changed: 591 ins; 315 del; 247 mod
  Patch: https://git.openjdk.java.net/lanai/pull/98.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/98/head:pull/98

PR: https://git.openjdk.java.net/lanai/pull/98