Allow specifying host images per hashed texture

[iwubcode]

Dolphin technically supports an individual list of host images per hashed texture. However, when we generate the JSON we generate the same host list for each hashed texture. It might be nice to be able to specify individually somehow.

This would allow for hash textures that have buttons with different styles, to be styled differently using different host images.

At the moment, the way a user would support this is creating multiple packs (or editing the Dolphin json).