RFR: 8342530: Specifying "@Nx" scaling level in ImageStorage should only load that specific level

[Lukasz Kostyra]

This follow-up change finishes the earlier changes to `ImageStorage.loadAll()` and adds support for loading specific scale requested in the input.

`loadAll()` will now first check if the input path ends with a scaling level specified, and if that is the case it will attempt creating a Stream. If requested resource does not exist it will throw an Exception, skipping the rest of the load process. If the resource does _not_ have a scaled name in its path, it will continue loading as normal - looking for all scale levels, trying to load the main resource and falling back to trying to load "@1x" variant.

Added tests to check the new `ImageTools.hasScaledName()` method + new behavior.

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Commit messages:
 - Add ImageStorage test
 - Add support for requesting specific scaling

Changes: https://git.openjdk.org/jfx/pull/1809/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1809&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342530
  Stats: 68 lines in 4 files changed: 60 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1809.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1809/head:pull/1809

PR: https://git.openjdk.org/jfx/pull/1809