Although I find the previous implementation more elegant, it results in
a deeper nesting of Promises than necessary, which can make debugging
more complicated. The canvas scaling and compression apis are actually
synchronous, so the callback structure isn't really recessary here.
Converting to a loop also makes this process easier to understand at
a glance.
Fixed some bugs along the way:
* accidentally scaling small images up to 1920px
* jpeg compressing gifs and other formats even if unnecessary
7c9ad975bb