Merge commit '2df72777'

* commit '2df72777':
  swscale: fix gbrap to gbrap alpha scaling

Conflicts:
	libswscale/input.c
	tests/ref/fate/filter-pixfmts-scale

Not merged, the code works fine as is and does not work with this
change
Merged-by: Michael Niedermayer <michaelni@gmx.at>