Fix no color/white signature

By trial and error I found that the -linear-stretch option of the convert command removes any color of the signature when placing it into the document.

$ convert --help
Version: ImageMagick 6.9.12-32 Q16 x86_64 2021-12-04 https://imagemagick.org

Example:

• original pdf
• falsisigned pdf without the linear stretch
• falsisigned pdf with the linear stretch
• the command run: ./signature/falsisign.sh -d prohlaseni-orig.pdf -s signature/signatures_blue -x 1200 -y 1645 -o prohlaseni-with-linear-stretch.pdf -S 80 (using the new -S option from !8 (closed))