(try to) distinguish between ProcessingError and EOSError.

problem is: av_read_frame returns -1 in both cases,
so we must inspect the nderlying AVIOContext.
No big deal, if url_feof() is public and does what promises :)

--HG--
branch : pure-cffi
factorize the mocks in one place

--HG--
branch : pure-cffi
nitpicking

--HG--
branch : pure-cffi
assertEquals -> assertEqual

--HG--
branch : pure-cffi
replace the hackish setup with a proper fixture.

--HG--
branch : pure-cffi
fix the raw packet access semantics.

Now is bearable: better names, safe(r) access while retaining
raw semantics, fixed all the demuxer bugs found so far, simpler code.
WIN?

--HG--
branch : pure-cffi
Merge ../pyrana