RFR: 8267565: Support "@1x" and "@3x" high-density image naming convention

[drmarmac]

This PR extends the range of hi-res images that are loaded via naming convention, now including scale factors higher than "@2x". 
Supporting these is already being [recommended](https://developer.apple.com/design/human-interface-guidelines/images#Best-practices) for some platforms.

I tested this manually on Windows with "300%" UI scale factor, and verified "@x2" still works on macOS.

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Commit messages:
 - Add extended high-dpi image support

Changes: https://git.openjdk.org/jfx/pull/1429/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8267565
  Stats: 49 lines in 5 files changed: 19 ins; 0 del; 30 mod
  Patch: https://git.openjdk.org/jfx/pull/1429.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1429/head:pull/1429

PR: https://git.openjdk.org/jfx/pull/1429