Detect unchanged image files and don't recompress

Imagecache will save and recompress files even if the actions didn't really change anything - i.e. a scale action didn't run because the size was right to begin with.

I created an ImageAPI issue that would indicate if the action changed the image in any way:

#737006: Add an 'UNCHANGED' return code for actions

And this is the Imagecache counterpart, detecting the IMAGEAPI_UNCHANGED value, and just copying the file if no changes were made. It requires the above patch to ImageAPI.