General information - GIF utilities
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  Each of the utilities provided in 'util' directory perform some kind of
filtering, generating new image, or dumping it to a device.

  Because they are not fast, a filter will print (in increasing) order the
current input image line number, any time it needs to read the input, and
will print output image line number (in decreasing order) any time it dumps
out. Other utilities, that only read or write, will always print that half
the same way (but in increasing order). This is true only for utilities
that decodes the data. Utilities (like GifPos that only change positions)
that copies the image as block of compressed data, will print nothing for
two reasons: they can not idetify line number, and mainly as they are much
faster.

  Some of the utilities requirs memory in the order of the whole screen, while
others read one scan line at a time. Each utility DOC file, has entry called
'Memory Usage' which will be one of: Line (memory required is in the order
of one scan line), Image (order of biggest image in GIF file), or Screen (order
of GIF file screen). In all cases, a byte is allocated per pixel, so image of
320 by 200 pixels will requires ~64k bytes to hold.