Improve multi-page TIFF/PDF picture import speed

When importing as a Picture a multi-page TIFF or PDF, the ImageMagick identify command is called to get the image size. However for a multi-page picture all the pages are read by ImageMagick even though we are only interested in the first one.

ImageMagick allows adding [0] at the end of the filename to specify that we're interested only in the first one (first frame in ImageMagick parlance).