Try gdImageCrop + gdImageScale instead of legacy gdImageCopyResampled

gdImageCopyResampled is retained for compatibility purposes, but in theory should be quite a bit slower than gdImageScale (which defaults to bilinear).

Also, gdImageSharpen is extremely inefficient, and should be avoided in favor of a composed scaling and sharpening weight kernel (which, unfortunately, isn't in the mainstream release yet).

vips-bench doesn't seem to work on OS X (`/usr/bin/time: illegal option -- f usage: time [-lp] command.`, and there are many warnings when linking against vips 7.42.1). I can't run the benchmark myself, but I think these changes may help.