Add APNG support

[pmrowla]

Fixes #3483.

Changes proposed in this pull request:

• Implements support for reading APNG files

  • seek() tell() can be used for accessing frames, seek() rewinding is supported
  • If an APNG contains a default image that is not part of the actual animation sequence (i.e. IDAT chunks without an fcTL chunk), im.info["default_image"] will be set and True, and the default image can still be accessed as frame 0 (seek(0)).
    In this case, the first animation frame will be at seek(1), and n_frames will contain the number of frames plus 1 to account for the default image.
  • If im.info["default_image"] is not present or is False it means the APNG did not contain a separate default image
  • im.is_animated will always be True for APNG files, even if the animation only contains a single frame, to differentiate single frame APNG files from standard PNG files.

• Implements partial support for writing APNG files

  • APNG animations can be written using save_all and append_images (same behavior as writing GIF sequences)
  • Loop and frame duration can also be specified (same behavior as GIF sequences)
  • APNG frame disposal method and frame blend method can be specified via disposal and blend parameters
  • All images from the append_frames sequence are expected to be the same size as the initial image, frame bounding box is calculated based on the differences between a given frame and the prior frame
  • Frame bounding boxes cannot be manually specified with this implemention

• Tests are based on the browser compatibility tests from https://philip.html5.org/tests/apng/tests.html, which are the only tests currently listed at: https://wiki.mozilla.org/APNG_Specification

[pmrowla]

I've been doing some more testing and found a problem in the code for writing APNG sequences that occurs if it has to write multiple fdAT chunks in a row for one image frame. I'll try to get a fix and new tests pushed for that at some point in the next couple of days.

[hugovk]

Some minor docs suggestions. Please could you also add release notes, linking to the docs for more info?