RFR: 8364768: JDK javax.imageio ImageWriters do not all flush the output stream

[Phil Race]

4 of the 6 JDK ImageWriters flush the stream at the end of writing. 2 (TIFF and GIF) do not. 

This will matter if you are using a caching ImageOutputStream.

This fix makes it consistent.

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Commit messages:
 - 8364768

Changes: https://git.openjdk.org/jdk/pull/26703/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26703&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364768
  Stats: 81 lines in 3 files changed: 81 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26703.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26703/head:pull/26703

PR: https://git.openjdk.org/jdk/pull/26703