WebP Translator update to 1.5 and header parsing issues

[grexe]

(can be seen as a followup to #13712 for the updating to 1.5 part)

Trying to read the attached valid WebP image file (opens fine in WebPositive) fails with the latest WebP translator from R1/Develop.

It also didn't load with the Google test decoder supplied with the official sources, because for some reason it is stored as an animation (?!).

However, by inspecting further, I noticed some things which I'd like to address in this issue:

1. error reporting for translators is very poor in general. I'll open a separate issue for this if you guys agree. At least, I should be able to know when a translator is the right fit for a particular file, but it cannot handle it for some other reason. Currently, this is all hidden from the user and it looks as if NO translator is installed for the filetype, which is wrong and leads to confusion.

1.the translator should be updated to consume the latest stable version of libwebp, latest update was half a year ago and the library builds just fine. See ​update libwebp to latest version 1.5.0

1. some error reporting would be nice, at least on stdout/stderr.
2. header parsing feeds a uin8 into WebP_GetInfo() but only consumes 64 bytes of the IOStream. Shouldn't it consume the full 256 bytes to fill the uint8 struct? Local testing this works but didn't help with the animation format, oc.

[grexe]

WebP image which is actually an animation