RFR: 8261620: Running metal with API validation crashes immediately

[Ajit Ghaisas]

Root cause : Trying to copy backbuffer-texture to frontbuffer-texture after accommodating for Insets causes some overflow which is detected only with Metal API validation.

Fix : Adjusted texture size to be copied to accommodate insets.

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Commit messages:
 - 8261620 fix validation error

Changes: https://git.openjdk.java.net/lanai/pull/184/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=184&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261620
  Stats: 14 lines in 1 file changed: 12 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/lanai/pull/184.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/184/head:pull/184

PR: https://git.openjdk.java.net/lanai/pull/184