prores: Reject negative run and level values

Sample-Id: 00000611-google

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
lavf/riff.c: Fix GeoVision H.264 fourcc.

Fixes d24da748
doc/filters/scale: do not explicitly state the default swscale flags value

This is done is order to avoid the need of synching scale with libswscale
docs in case the libswscale default changes.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  prores: Reject negative run and level values

See: 93c220e895a417e88a6be7df85e250c5c3f675b6
Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/codecs: document skip_alpha

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>