If /usr/local/lib is in the search path, a Homebrew installed libJPEG
can end up conflicting with the system libJPEG as such:
```
$ otool -L /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
/System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO:
... snip ...
/System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libJPEG.dylib (compatibility version 1.0.0, current version 1.0.0)
... snip ...
```
```
$ cargo clean && cargo test
... snip ...
Doc-tests magick_rust
dyld: Symbol not found: __cg_jpeg_resync_to_restart
Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
Expected in: /usr/local/lib/libJPEG.dylib
in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
```
Removing the /usr/local/lib link search path allows successful
compilation and testing on my OSX 10.11.6, with jpeg 8d installed into
/usr/local/lib via Homebrew. I realize this might be a problem with my
setup and not a universal issue, but as Homebrew is so common I thought
I'd submit this patch anyways.
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.
Pete Gadomski