RFR: JDK-8210547 synchronise with OpenGL after the last window is presented

[notzed]

As discussed on the mailing list.

OpenGL requires an explicit glFinish() to synchronise with the video refresh.  The current usage is incorrect and against the specification.  This patch calls glFinish() after all windows have been drawn following a pulse.

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Commit messages:
 - JDK-8210547 synchronise with OpenGL after the last window is presented

Changes: https://git.openjdk.org/jfx/pull/1981/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1981&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8210547
  Stats: 29 lines in 5 files changed: 26 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1981.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1981/head:pull/1981

PR: https://git.openjdk.org/jfx/pull/1981