Make imagemagick identification more robust

1)  Filenames might have spaces, so we should quote the filename.
    Escaping the spaces or putting quotes in the datafile itself wreaks
    havoc elsewhere in the code, pretty much everywhere Perl checks
    whether a file exists. Bad.

2)  GraphicsMagick's version of `identify`, at least, gives me output
    like "JPEG 775x2120+0+0", i.e. without a space after the second
    dimension, which doesn't match the regex. I'm not sure whether this
    is just an incompatibility between GraphicsMagick and ImageMagick,
    or whether newer versions of ImageMagick do this too. In any case
    I think it can't hurt to change the regex the way I've done in this
    commit.