Fail to decode after printing: Error: cannot find upper left corner

I'm printing a file that fits on one single A4 page. The printed version looks good.

However, when decoding after scanning, I hit `Error: cannot find upper left corner`

Any idea to fix that?

Thanks a lot for the great project! --Martin

```
$ ./unoptar 0-65-93-24-3-1-2-24 02_sound > foo
Format: text height=24
Unformatted channel capacity 433.267 kB, formatted raw channel capacity 399.546 kB, net Golay payload capacity 199.773 kB, 133182 Golay symbols, 0 bits unused (incomplete Hamming symbol), border taking 1.50439% of unformatted capacity, border with crosses taking 7.78285% of unformatted capacity, border with crosses and Golay taking 53.8914% of unformatted capacity.
Decoding PNG file 02_sound_0001.png...
Input 595 x 841 pixels, taking 1.00079 megabytes for 2 framebuffers.
Average pixel value 226
Black 40.3372, white 254.53, cutlevel 62 (0x3e), fill cutlevel 147 (0x93)
Black 16.6485, white 251.518, cutlevel 40 (0x28), fill cutlevel 134 (0x86)
Black 12.8823, white 249.818, cutlevel 37 (0x25), fill cutlevel 131 (0x83)
Black 12.3657, white 249.524, cutlevel 36 (0x24), fill cutlevel 131 (0x83)
Black 12.2164, white 249.445, cutlevel 36 (0x24), fill cutlevel 131 (0x83)
Removing dirt from the white border: white border identified, data area identified, erased 64496 pixels of dirt.
Searching for the corners.
Error: cannot find upper left corner
See failure_debug.pgm why.

```