raise exception, do not assert
review and simplify the pygame example.
the obvious typo
fix a nasty crasher bug: allow access to image data.

Yeah, it took WAY too much to finally remember that chroma subsampling
works BOTH in width and height. I got entangled in the tought that the
problem was a mismatch betwwen bytewidth, width and linesize, instead
this was right (almost) since the beginning; I just left the height
buggy.
nitpicking
simplify and correct the code. Still WIP.