RFR: 8263778: Fix monocle JNI signatures for instance methods

[Johan Vos]

Fix signatures (jclass/jobject), add UNUSED when unused
Fix for JDK-8263778

The monocle/egl native code now includes the javac generated header file
UNUSED() directives are used when a parameter is not used
jobject/jclass are used when expected.

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Commit messages:
 - Fix signatures (jclass/jobject), add UNUSED when unused

Changes: https://git.openjdk.java.net/jfx/pull/435/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=435&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263778
  Stats: 22 lines in 1 file changed: 2 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/jfx/pull/435.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/435/head:pull/435

PR: https://git.openjdk.java.net/jfx/pull/435