Add a gradient to FancyCaptcha

This defeats naive thresholding, giving Tesseract break rate of 0 out of
1000, even if a sensible threshold value is hand-chosen. Reduced the
text value and noise to make room for the gradient, but kept an SNR of
1.3, as before, which provides good legibility.

Obviously the gradient can be removed with custom preprocessing -- the
point of these changes is to raise the bar from "unconfigured Tessearct"
to "some small amount of developer effort".

Change-Id: I30ebc904ca59bf29a2aa812f881a077a13493e68