Fixes for new ImageMagick:

Fixes for new ImageMagick:
* (bug 24824) Support ImageMagick 6.5.6-2+ JPEG decoder size hint, to reduce memory usage when such an ImageMagick is used for scaling.
* Removed -size option since it doesn't act as a JPEG decoder size hint with recent ImageMagicks, and it may cause undesired behaviour in the future since it's documented to do something different now.
* Disable multithreaded behaviour in recent ImageMagick, to avoid a deadlock when a resource limit such as $wgMaxShellMemory is hit.
* Fixed some comments.

Replaces WMF live hack.