RFR: 8306707: Support pluggable image loading via javax.imageio [v8]

[Michael Strauß]

> This PR is an improved version of #1093.
> 
> JavaFX can load BMP, GIF, PNG, and JPEG images with its built-in image loaders. It has been a long-standing request to support more image formats, most notably (but not limited to) SVG. However, adding more built-in image loaders is a significant effort not only in creating the functionality, but also in maintaining the additional dependencies.
> 
> This will probably not happen any time soon, so we are left with three alternatives:
> 1. Accept the fact that JavaFX will never be able to load additional image formats.
> 2. Create a public image loader API, and hope that developers in the JavaFX ecosystem will create image loader plugins.
> 3. Leverage the existing Java Image I/O API.
> 
> From these options, I think we should simply support existing Java APIs; both because it is the shortest and most realistic path forward, but also because I don't think it is sensible to bifurcate pluggable image loading in the Java ecosystem.
> 
> Of course, Java Image I/O is a part of the `java.desktop` module, which as of now, all JavaFX applications require. However, it has been noted in the previous PR that we shouldn't lock JavaFX into the `java.desktop` dependency even further.
> 
> I've improved this PR to not permanently require the `java.desktop` dependency: if the module is present, then JavaFX will use Image I/O for image formats that it can't load with the built-in loaders; if the module is not present, only the built-in loaders are available.
> 
> I have prepared a small sample application that showcases how the feature can be used to load SVG images in a JavaFX application: https://github.com/mstr2/jfx-imageio-sample

Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Merge branch 'master' into feature/ximageloader
 - review changes
 - scanline stride always measured in bytes
 - remove unused code
 - fix line endings
 - fix line endings
 - add support for palette-based image formats
 - review changes
 - review changes
 - doc changes
 - ... and 10 more: https://git.openjdk.org/jfx/compare/0091d9c3...bcc4f196

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1593/files
  - new: https://git.openjdk.org/jfx/pull/1593/files/40e960dd..bcc4f196

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1593&range=07
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1593&range=06-07

  Stats: 1872 lines in 83 files changed: 1240 ins; 308 del; 324 mod
  Patch: https://git.openjdk.org/jfx/pull/1593.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1593/head:pull/1593

PR: https://git.openjdk.org/jfx/pull/1593