RFR: 8361286: Allow enabling of background loading for images loaded from an InputStream

[John Hendrikx]

Support background loading of raw input streams

- Fixed generics (mix up of two ImageLoader types)
- Removed unused code for handling headers, methods, request parameters
- Use `long` for progress as streams may exceed 2 GB
- Improved documentation of Image regarding background loading

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Commit messages:
 - Support background loading of raw input streams

Changes: https://git.openjdk.org/jfx/pull/1875/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1875&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361286
  Stats: 304 lines in 11 files changed: 180 ins; 72 del; 52 mod
  Patch: https://git.openjdk.org/jfx/pull/1875.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1875/head:pull/1875

PR: https://git.openjdk.org/jfx/pull/1875