This method goes down to `MagickGetImagesBlob`, which allows for
creating animated gifs.
I didn't abstract out common operations between the two in the interest
of not overly polluting the codebase. However, this new method is almost
identical to `write_image_blob` so you could probably abstract something
out of there.
The test image that was meant to be rotated was generated incorrectly.
It now corresponds to the following ImageMagick command:
$ convert in -rotate 90 -orient left-bottom out
cargo test passes