[jfx25u] RFR: 8368631: Avoid updating disposed MTLTexture

[Ambarish Rapte]

This is a metal specific safe change. This fixes a crash issue that occurs when a Texture is updated after disposal.

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Commit messages:
 - Backport c480b4622e7a1dc4e1987649eedf972103380b61

Changes: https://git.openjdk.org/jfx25u/pull/32/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx25u&pr=32&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/jfx25u/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/jfx25u.git pull/32/head:pull/32

PR: https://git.openjdk.org/jfx25u/pull/32