Specify how parallel image fetching works.
Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24711
Also:
* Fire error event when URL fails to parse.
  Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=26076
* Fire error event instead of load event when image sniffs as an image but is broken.
  Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=26057
* Make img be "unavailable" until dimensions are known or it's broken.
  Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25891
* Fire 'progress' event only for the current request.
  Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25795
* Specify how 'list of available images' and HTTP cache interact.
  Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25796