Support for EXT_texture_webp

[hannojg]

Hello,

I understand that not all EXT_ are supported yet. I am particularly interested in support for the EXT_texture_webp extension. Do you want to support this extension in cgltf? If so, I could check to contribute 😊

[jkuhlmann]

Hi! This one seems pretty straightforward and could probably easily be accessed through the generic extension pointers. However, if you feel like contributing it, I'll be happy to merge it. 😀

[bobbens]

Implemented in #253, seems to work here.

[hannojg]

Thanks for the work guys!