Integrate APNG into show and save methods

[gagern]

Once #16571 and #7298 get merged, it is time to add support for animapted PNGs (APNG) to the Animation.show() and Animation.save() methods, and also to update the doctests to use show(format="png") for these.

[gagern]

Only the last commit is really new. The merge before that did resolve a conflict, but in a pretty obvious way. Everything else is from the named dependencies.

---

Last 10 new commits:

​e2d3e11	Proper hyperlink formatting in animate documentation.
​bd17f04	Add APNG support.
​a60fe0b	Turn generator into list when rendering frames.
​04665b0	Fix frame size for standard example animation.
​f4a2427	Merge fix of standard example animation into APNG branch.
​8af6a08	Base Animation.show on Animation.save, and pass more arguments.
​3beb8b1	Create HTML to embed generated GIF in notebook.
​c16f275	Support HTML5 video tag.
​effa6cf	Merge branch ticket/7298 into ticket/16571 to create ticket/16650.
​83d734c	Add support for APNG to Animation.save().

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​bc9f681	Merge fix of standard example animation into APNG branch.
​780e614	Proper hyperlink formatting in animate documentation.
​10f8e52	Make number of frames available after rendering using generator.
​941170b	Add APNG support.
​3a99225	Merge ticket/16645 into ticket/16571 to deal with conflict.
​f0bfc49	Allow embedding animations into tables.
​2fb3d6a	Mark test saving GIF as optional.
​f61d9d1	Merge branch ticket/7298 into ticket/16571 to create ticket/16650.
​1e4de99	Add support for APNG to Animation.save().

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​e216b92	Make number of frames available after rendering using generator.
​b057e61	Add APNG support.
​f347d4c	Doctests for each method of APngAssembler using tracing fixture.
​7d2531d	Details on my APNG contribution in module documentation.
​dc0067d	Base Animation.show on Animation.save, and pass more arguments.
​4e9101f	Create HTML to embed generated GIF in notebook.
​e4807cc	Support HTML5 video tag.
​2f6a670	Allow embedding animations into tables.
​d0e89d5	Merge branch ticket/7298 into ticket/16571 to create ticket/16650.
​f5fc857	Add support for APNG to Animation.save().

[gagern]

The recent changes from #17234 prevent our dependency #7298 from being merged, and at the moment I'm far from sure how to rebase that code, see comment:48:ticket:7298. Once I get that rebased, there is one question regarding this ticket here as well:

Should we reuse the PNG format description, or introduce a new one?

On the one hand, APNG (as opposed to MNG) is very much about graceful degradation, so it makes sense to treat it as PNG in most places. On the other hand, perhaps the display manager can decide whether a client is actually capable of displaying APNG or not. If so, we might prefer supported APNG over GIF, but GIF over unsupported APNG interpreted as still PNG. If the display manager does not know about APNG support, but does know that PNG is supported, then the user should at least be able to choose APNG format manually, even though it might not be the auto-selected default in that case.

[vbraun]

Do not reuse OutputImagePng, that is for non-animated png.

Another basic output format should be a sequence of PNG, which can easily be displayed in a web browser with javascript (embed images as necessary). Perhaps the easiest animation format.

[gagern]

Replying to vbraun:

Another basic output format should be a sequence of PNG, which can easily be displayed in a web browser with javascript (embed images as necessary). Perhaps the easiest animation format.

I'm not sure how much such a format would suffer from HTTP overhead, particularly for a large number of small frames. I guess it depends on the network connection (for the cloud in particular) whether that factor outweights the overhead of concatenating all PNGs to a single APNG file.

But if you are willing to use custom JavaScript code for animations, you could also use ​existing code which provides APNG support on browsers which don't support it natively.

[vbraun]

I'm thinking about base64-encoded inline images, especially since IPython doesn't allow you to return arbitrary collections of files. Though an apng polyfill would work, too.

[gagern]

I intend not to work on this until #7298 gets reviewed and ready for merge.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​a8d97b7	Trac #7298: Enable HTML5 video in the Sage Notebook
​f6e6412	Trac #7298: Implement output types for various video container formats
​ea5dfe2	Trac #7298: Drop support for the autoplay and controls attributes for video
​de1dcbb	Add support for APNG to Animation.save() and Animation.show(format="png").

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​6c6eee3

Trac #16650: Add support for APNG to Animation.save() and Animation.show().

[gagern]

Since #7298 received a positive review just now, here is a rebased version of this modification here. With the new rich representations framework, there was a lot more to do than in the original commits. But it integrates APNG nicely with all the rest.

[chapoton]

does not apply