Adjust IDENTIFY_OUTPUT_RE for graphicsmagick

The 'identify' binary supplied in graphicsmagick-imagemagick-compat outputs
lines of the form

     fontchars/font033.gif GIF 9x16+0+0 PseudoClass 32c 8-bit 194 0.000u 0:01

However the IDENTIFY_OUTPUT_RE regex was written against an output which lacked
the +0+0 suffix to the dimensions. This patch adjusts accordingly.