Add support for Windows about splitwebp

[oktak]

Hi! I would like to try this. But it is a challenging on Windows. I am using:

Win10 (x64)
VS Community 2017 (15)
Cmake 3.18.3
OpenCV 3.4.11
libwebp-1.1.0-windows-x64.zip  

I can successfully compile (I guess my deliverable for this issue would be a README, loads of un-scriptable steps involved 🙄 ) into a splitwebp.exe. But when I try to passing a webp file to run it, it has no output files, and also shows no error message on the CMD.

I tried these animated webp files:

1. https://mathiasbynens.be/demo/animated-webp
2. http://littlesvr.ca/apng/gif_apng_webp2.html
3. https://ezgif.com/webp-maker/ezgif-6-53fa07a0-webp

So, I started to debug mode. I modified main.cpp and hard code a image path on disk.

It shows an Exception: Access violation reading location. In fact, in current_frame_iter can parse the correct details about the webp file. Only the fragment.bytes and fragment.size property got wrong.

I tried all webp files above. Then I tried to edit this line if (WebPDemuxGetFrame(demux, 1, &current_frame_iter)) {
I changed it to if (WebPDemuxGetFrame(demux, 2, &current_frame_iter)) {

Then it no longer popup a Exception, but current_decoded_frame become NULL pointer and it return in line 103.

I did not tried to compile on Linux or Mac. (But I learnt that a user has successfully compiled on Linux in Issue #3 discussion)
To summarize the issues I face, could you help:

1. Specify the library dependencies version, so that we could follow that,
2. Offer some animated webp files for testing purpose?

Thanks!

[Razdeep]

Hi! Could you try on this animation?
https://splitwebp.s3-eu-west-1.amazonaws.com/angry.webp

[Razdeep]

@oktak you can add the steps for building in windows in the README

[oktak]

I've added the steps for building in windows. I put it in a separated file because it is quite long.

I also tried the animation you mentioned. It still give the same Exception: Access violation reading location. I am still figuring out what happens. Is it related to pthread? It seems Windows does not have pthread, so it remove it in the CMakeLists.txt . Please let me know if it is a must.

[Razdeep]

pthread was a dependency on libwebp in Linux.
I was getting an error, without linking it.
It might be possible that missing pthread is causing the problem.
I'm not sure about this. I haven't tried it on windows yet.

[oktak]

I see. I find that vcpkg which is Microsoft's product to help mange packages for Visual Studio. I find people us it to install pthread for windows and also libwebp.

After some configuration, the Exception: Access violation now goes deeper, it locates at WebP.dll

I try to explore on that.

[oktak]

Thank you for your review on my Pull Request. Sorry for my typo mistakes.

Updates:
I found it is probably the problem of include files of webp (decode.h, demux.h, ...etc).
It looks the the include files are in around version libwebp-0.4.1-windows-x64, but the version I tried before is libwebp-1.1.0-windows-x64

When I set the linker to use the include folder using the one of vcpkg, it can now produce the images of each frame.
(though the filenames are a bit long 😬 Updates: Oh! Just found if I use the "Release" config, it will be start from 1, without leading zeros.)

I'll update the README on both your suggestions and adjust the steps on building on Windows.