Improve mipmap computation

[ensisoft]

glGenerateMipmap has just way too many caveats and limitations regarding sRGB textures.

So this means we'd need to generate the mip maps ourselves.

One option is For textures that are known at packaging time we can already compute the mips while packaging but this does create larger packages.

Other option is to add a preload step see #176 and upload mips at this point