RFR: 8361286: Allow enabling of background loading for images loaded from an InputStream [v6]

[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

John Hendrikx has updated the pull request incrementally with two additional commits since the last revision:

 - Clarify docs
 - Fix formatting

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1875/files
  - new: https://git.openjdk.org/jfx/pull/1875/files/6494c52a..158d1781

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1875&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1875&range=04-05

  Stats: 7 lines in 2 files changed: 0 ins; 0 del; 7 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