process a crop as a resize when possible

Currently dokuwiki will generate two cached images when both width & height for an image in the wiki, a 'crop' then a 'resize'.  When the aspect ratio of the crop matches the aspect ratio of the source image, it is effectively the equivalent of a resize.

This patches adds a check in the crop function to see if the final image dimensions would match those of a resize and if so, calls resize immediately rather than after the crop, resulting in one cached image.

e.g.  source image.jpg  2574px x 1926px
image.jpg?1000
image.jpg?1000x748
will now both use the same cached image, cache/hash.media.1000x748.jpg
rather than the previous three images
cache/hash.media.1000x748.jpg
cache/hash.media.2574x1926.crop.jpg
cache/hash2.media.1000x748.jpg