Surface the ImageLoader API

[Michael Strauß]

I've been trying to add SVG support to a JavaFX application (which is
the image format I'm encountering most often in app projects), but
that's quite hard because the ImageLoader API is pretty much closed
for extension.

I'm proposing to open up the ImageLoader API to enable third-party
developers to create image loaders for emerging image formats like SVG
or WebP.

There will need to be some minor API and behavioral changes:


1. `ImageLoader.load` requires a new parameter "pixelScale":
current: ImageFrame load(int, int width, int height, boolean, boolean)
new: ImageFrame load(int, int width, int height, double pixelScale,
boolean, boolean)

This parameter is important for vector image loaders like SVG. If an
image loader is invoked with `width==0` and `height==0`, the
implementation should return an image with its natural resolution.
Vector image loaders need to know the requested pixel scale in order
to render the image with the correct pixel density.


2. `ImageFormatDescription.Signature` must be able to accept `null`
values in the byte array:
current: public Signature(byte...) {...}
new: public Signature(Byte...) {...}

A `null` value will match any byte at the corresponding position in
the image file header. This is necessary to match file headers that
contain variable data, like the WebP image header:
byte 0-3: 'R', 'I', 'F', 'F'
byte 4-7: <file size>
byte 8-11: 'W', 'E', 'B', 'P'

Note that in this example, the corresponding signature description would be:
{ 'R', 'I', 'F', 'F', null, null, null, null, 'W', 'E', 'B', 'P' }


3. Enable file extension matching (in addition to signature matching)
with the following semantics:
Any valid `ImageFormatDescription` MUST contain at least one file
extension, and MAY contain any number of signatures and MIME types (or
none at all). When `ImageStorage` chooses an image loader for a file,
it first tries to match the file by signature, and if it doesn't
match, it tries to match by file extension (or, if the source is a
"data" URI, it tries to match by MIME type).

The reason for this is that some image formats like SVG cannot be
meaningfully matched by signature; in this case, the SVG image format
description would not contain a signature description and would always
fall back to matching by file extension or MIME type.


I'd welcome any opinions on this proposal.