Various fixes to ImageResource processing algorithms

[mgiuca]

This change (choose one):

• Breaks existing normative behavior (please add label "breaking")
• Adds new normative requirements
• Adds new normative recommendations or optional items
• Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
• Is a "chore" (metadata, formatting, fixing warnings, etc).

Commit message:

Normative:

• Removed passing the name of the member to the processing algorithm. It
  just directly takes the array of image resources. This fixes a
  mismatch between the callers (which were already passing the array)
  and the definition of the algorithm.
• Removed definition of steps for processing "src". This was doing
  almost nothing; just inline the parsing of the URL.
• Copy "platform" into the output array.

Formatting:

• Fixed links to "sizes" definition.
• Remove redundant list in processing algorithm.

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Preview | Diff

[mgiuca]

@aarongustafson Please have a look at this.

I noticed so many issues while reviewing w3c/image-resource#6 which were copied from here, that I think we should fix them upstream first (since they won't be deleted from Manifest for some time).

If you wouldn't mind, can you copy the changes into w3c/image-resource#6 as well? (The commit description summarizes all the changes which you could make by hand.)

[aarongustafson]

LGTM. Given that most of this is going to be backed out once ImageResource is ready, I don’t think we need to get super particular about the naming of manifest URL/base URL vs. context, etc.