Make mipmap generation faster

[RunDevelopment]

As the discussion in #18 has shown, the current mipmap generation leaves a lot of performance on the table.

In particular:

• Nearest is very slow. (Faster mipmap generation with Nearest resizing #106)
• U8 for almost all filters is between 2x and 3x slower than FIR.

[RunDevelopment]

Change of plan.

Instead of trying to manually implement optimized resizing algorithms, this crate should just use a faster library. Since image recently switched to PSS, I think it would be a good idea to do the same. I already started making NN faster for PPS, so I may as well try to improve performance further over there and have it trickle down to both this crate and image.