Use a 4-channel f32 pixel intermediate type for resizing.
Use a 4-channel f32 pixel intermediate type for resizing.

This maintains precision and avoids a lot of unnecessary rounding to
integers for intermediate values. This improves precision and
performance.

On a 2950x:

Before: test imageops::sample::tests::bench_resize                     ... bench:   3,179,804 ns/iter (+/- 129,332) = 641 MB/s
After: test imageops::sample::tests::bench_resize                     ... bench:   1,563,759 ns/iter (+/- 87,143) = 1304 MB/s

PistonDevelopers/image,PistonDevelopers/image