ImageMagick::convert fails on converting image to different format

Problem/Motivation

ImageMagick::convert fails if a different destination image format is requested via ::setDestinationFormat.
This is because the destination file path is prepended with 'format:' which fails the file_exists check after conversion.

Proposed resolution

Move the prepending of the image format to the ::imagemagickExec method, leaving the destination file path untouched.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.