[OpenJDK 2D-Dev] RFR: 8263488: Verify CWarningWindow works with metal rendering pipeline

[Ajit Ghaisas]

Root cause : 
CWarningWindow creates a MTLLayer with null peer.
In MTLLayer.replaceSurfaceData() method, insets should be set only if peer is not null.

Fix : 
Added the required null check.

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Commit messages:
 - set native insets only if peer is not null

Changes: https://git.openjdk.java.net/jdk/pull/3086/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3086&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263488
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3086.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3086/head:pull/3086

PR: https://git.openjdk.java.net/jdk/pull/3086