sws: factorize rgb 9+ planar funcs declarations.
lavfi/alphaextract: copy width and not linesize.

Padding data is not supposed to be copied.
lavfi/overlay: reindent fix.
gifdec: reset previous Graphic Control Extension disposal type

This fixes out of array accesses.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>