Add maketx option to sharpen as we create MIP levels.

I know this might be contentious (and wrong in so many ways), but bear with me.

The idea is that production sometimes thinks the filtered textures look a little too blurry and washed out (even though we know that they're filtered almost exactly right). So with this, we optionally do a sharpening step (via unsharp_mask) as we create MIP levels. This gooses the high-frequency detail just a bit.  Example command line:

    maketx input.exr -sharpen 0.5 -filter unsharp-median -o texture.exr

Play with the sharpening amount (I like somewhere between 0.25 and 0.5) and with whether you use the unsharp-median or (by leaving out the -filter or setting it to an ordinary name like "lanczos3") whether you use the usual Gaussian-based unsharp mask

This is all experimental! Use at your own risk.

I'm not ready to advocate using this yet... it's going in for experimental purposes, but we'll let you know if our productions like it and you can make up your own minds.