Investigate faster mipmap generation

[RunDevelopment]

Right now, I'm always downsampling from level 0. This creates a lot of redundant work as the mipmap for level n could be generated from level n-1. However, I suspect that:

1. This might only be possible for images where the dimensions are powers of 2.
2. Filters like Lanczos might produce different results.
3. Quantization error might accumulate and noticeable affect results.

Investigate whether this is feasible. Box is the default filter, so it would be a win if just this one filter could be faster.