RFR: 8374340: FontRenderContext instance variables should be final

[Daniel Gredler]

Instances of FontRenderContext are immutable, but its instance variables are not declared final.

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Commit messages:
 - Make FontRenderContext instance variables final

Changes: https://git.openjdk.org/jdk/pull/28981/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28981&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374340
  Stats: 14 lines in 1 file changed: 7 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/28981.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28981/head:pull/28981

PR: https://git.openjdk.org/jdk/pull/28981