RFR: 8329098: Support "@1x" image naming convention as fallback

[Lukasz Kostyra]

This PR adds a title-mentioned fallback to `ImageStorage.java` and a test for that specific scenario.

In `ImageStorage.loadAll()` we still prefer the Image path that was provided by the user, but if it is missing we will now try to add a `@1x` suffix to Image name and give it one last try.

Added test includes a binary file `checkers at 1x.png`. This was added to differentiate it from the collection of `checker.png` files and their higher scale variants, but it is a direct copy of already existing `checker.png` test resource, just with a different name and suffix. Test should fail without a change in `ImageStorage.java` and pass with it.

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Commit messages:
 - Add test for 1x image fallback
 - Add fallback for @1x image naming

Changes: https://git.openjdk.org/jfx/pull/1598/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329098
  Stats: 20 lines in 3 files changed: 18 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1598.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1598/head:pull/1598

PR: https://git.openjdk.org/jfx/pull/1598