Thumbnails by width and height are obtained by a cropping operation
(which best preserves either the height or the width of the original
image), followed by a scaling operation to the given h and w.
Test examples:
http://localhost:8080/roda/admin/thumbnail/alias/login/h/300/w/300
http://localhost:8080/roda/admin/thumbnail/alias/login/h/200/w/100
http://localhost:8080/roda/admin/thumbnail/alias/login/h/100/w/200