Visible signature image

[Tilman Hausherr]

I believe that somebody mentioned somewhere that creating the signature 
image didn't work properly, but I just can't find out who it was. While 
working on a test for JPEGFactory (PDFBOX-1969) I noticed that 
JPEGFactory.createFromImage() was temporarly broken (now hopefully no 
more), and this method is only used by 
PDVisibleSigBuilder.createSignatureImage().

I see now that this was created in PDFBOX-1766 by Thomas and Vakhtang - 
please test whether it still works.

Tilman