RFR: 8368631: Avoid updating disposed MTLTexture

[Ambarish Rapte]

Issue: 
Calling MTLTexture.update() method on a disposed MTLTexture object would cause a crash.
The root cause of this behavior is [JDK-8368629](https://bugs.openjdk.org/browse/JDK-8368629).

Fix:
Avoid MTLTexture.update() if texture is disposed.
The crash should be fixed now and the root cause will be followed up using [JDK-8368629](https://bugs.openjdk.org/browse/JDK-8368629)

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Commit messages:
 - avoid MTLTexture.update if disposed

Changes: https://git.openjdk.org/jfx/pull/1919/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1919&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368631
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1919.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1919/head:pull/1919

PR: https://git.openjdk.org/jfx/pull/1919