RFR: JDK-8302281 - ImageIO objects should implement AutoClosable

[Markus KARG]

ImageIO objects, like ImageReader, ImageWrite, ImageInputStream and ImageOutputStream, should implement AutoClosable.

There is a risk of keeping resources opened longer than needed (or even run into a resource leak with long running processes) as the user cannot use try-with-resources currently. The fact that there is a `dispose()` method to be called is easily missed, as people tend to look for `close()`.

This is not a big change, but it is useful for those working with ImageIO.

-------------

Commit messages:
 - Implements AutoCloseable

Changes: https://git.openjdk.org/jdk/pull/12098/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12098&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302281
  Stats: 20 lines in 3 files changed: 14 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12098.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12098/head:pull/12098

PR: https://git.openjdk.org/jdk/pull/12098