iOS support

[timleader]

Setup build scripts for iOS

[TimAidley]

You probably know this already, but if you try and build with iOS, it fails due to trying to access snappy-unity3d.

[timleader]

Just pushed a potential fix for it, as you might have guessed one of the build scripts for iOS was a direct copy from snappy-unity3d and was not yet amended for webp-unity3d, unfortunately I'm not able to test this at the moment. Let me know if this works for you or not. Thanks for your help.

[TimAidley]

Thanks for the alterations; however that script suffers from not supporting identically named files in different subdirectories. Webp has several identically names files in both enc/ and dec/ directories, for instance. I'll see if I can come up with a fix.