Allow custom images

[JannikGM]

I think it would be nice if the user could select a custom base image to experiment how this affects real-world cases.
Ideally there'd be an option to generate the mip-chain, and another to actually load each mip level explicitly.

For example, the game "The Legend of Zelda: Wind Waker" on GameCube used explicit control over mipmaps to achieve an artistic effect (fading water waves in distance).

Another example, would be maps of cities, which is a common application of WebGL. It might be interesting to see how mipmaps affect labels / text on the map (here, it might make more sense to generate the mipmap chain automatically).