WebP support

[inorichi]

This PR adds support for WebP images (animated too) and updates gradle and the SDK to the latest version available.

Motivation: some sites may be using webp images, and Skia already supports this, so I thought why not include it here too. I'll accept it if you don't want to support webp though :).

I've split this PR into as many commits as possible to help reviewing. I can remove some of them if desired or move to a different PR.

This PR uses the whole libwebp library (tag v0.6.1). I haven't found a "decoding only" fork, but the folders enc and mux could be deleted alongside some files inside dsp and utils. Those are not compiled anyways so it's probably not a problem.

The complete function in image_webp is unfinished, but I don't understand yet how it works (I saw in png you clear the frames' buffer starting from the second frame and start over, but I still don't understand it).

Also, I'm a bit concerned about memory management in webp_decode but I think it's fine.

[seven332]

It may take seconds to decode an animated image. That make users wait too much time.

My solution

1. decode the first frame
2. display the first frame
3. decode other frames
4. start animation

The step 3 is the complete function.

[seven332]

Please open a separate PR to update gradle and android sdk.

[inorichi]

Done.

I'll try later to finish the complete function, and I can also give a try to #3 while I'm at it (in a separate PR).

[msm595]

Any updates with this PR?

[inorichi]

I'm not working on it yet because the image structures are going to change (#14) and the reply I got on #12.

Feel free to take over this PR, after all you should know better than me how to integrate it with the modular approach.

[inorichi]

I ended up rebasing the PR, however performance in release mode is 2x slower than Skia which also uses libwebp, so I think neon optimizations or cpu features are not working properly.

I've tried to improve performance but the result was always the same. This also happened with the old Android.mk build instead of CMake... I'm out of ideas.

[seven332]

It helps.

bool webp_decode_buffer(...) {
    ...
    webp_config.options.use_cropping = clip;
    ...
    webp_config.options.use_scaling = ratio != 1;
    ...
}

[inorichi]

Indeed... that was the reason, now I've got the same results as Skia. Thanks!

[seven332]

NEON is enabled on armeabi-v7a without runtime detection. But not all armeabi-v7a CPUs support NEON. Adding a cflag WEBP_HAVE_NEON_RTCD should be enough.

Please check here and here.

[arkon]

Was there any plans to merge this in?