[jfx17u] RFR: 8260528: Clean glass-gtk sizing and positioning code

[Jose Pereda]

Clean backport of 8260528: Clean glass-gtk sizing and positioning code
Reviewed-by: jvos, kcr

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Commit messages:
 - 8260528: Clean glass-gtk sizing and positioning code

Changes: https://git.openjdk.org/jfx17u/pull/172/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx17u&pr=172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8260528
  Stats: 750 lines in 5 files changed: 175 ins; 434 del; 141 mod
  Patch: https://git.openjdk.org/jfx17u/pull/172.diff
  Fetch: git fetch https://git.openjdk.org/jfx17u.git pull/172/head:pull/172

PR: https://git.openjdk.org/jfx17u/pull/172