RFR: 8365467: Fix multiple issues in ExplodedImage [v2]

[David Beaumont]

> This PR addresses several issues found while adding unit tests for ExplodedImage.
> I have added review comments for changes (some of which are a little preferential, but bring the code into line with things like ImageReader in terms of the name choices for variables).
> Later it is likely that this code will be adapted for the up-coming preview mode support in Valhalla, so having it unit-testable is important.

David Beaumont has updated the pull request incrementally with one additional commit since the last revision:

  Feedback tweaks.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26757/files
  - new: https://git.openjdk.org/jdk/pull/26757/files/1408def0..51a46c8f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26757&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26757&range=00-01

  Stats: 16 lines in 1 file changed: 6 ins; 1 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/26757.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26757/head:pull/26757

PR: https://git.openjdk.org/jdk/pull/26757