Center the resized image on the canvas, instead of shifting it slightly
Improve the way the new canvas size is calculated

If both dimensions are given, always use those dimensions, avoiding roundoff
error.

If only one dimension is given, slightly prefer rounding the other dimension
up, to rounding it down.
Use range for more idiomatic Go
Make kernel the only dependency of convolution1d
Merge branch 'improve-canvas-size' of http://github.com/jsummers/nfnt.resize into fix-canvas-size-issue
Test for correct canvas size.
Merge branch 'no-image-shift' of http://github.com/jsummers/nfnt.resize
Merge branch 'fix-canvas-size-issue'