[jfx17u] RFR: 8275723: Crash on macOS 12 in GlassRunnable::dealloc

[Kevin Rushforth]

Clean backport. Tested on macOS 10.15.7 and 12.0.1-beta.

-------------

Commit messages:
 - 8275723: Crash on macOS 12 in GlassRunnable::dealloc

Changes: https://git.openjdk.java.net/jfx17u/pull/18/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx17u&pr=18&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275723
  Stats: 18 lines in 1 file changed: 4 ins; 13 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx17u/pull/18.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx17u pull/18/head:pull/18

PR: https://git.openjdk.java.net/jfx17u/pull/18