avcodec/apng: Dispose previous frame properly

The spec specifies the dispose operation as how the current (i.e., currently
being rendered) frame should be disposed when the next frame is blended onto it

This is contrary to ffmpeg's current behaviour of interpreting the dispose
operation as how the previous (i.e., already rendered) frame should be disposed

This patch fixes ffmpeg's behaviour to match those of the spec, which involved
a rewrite of the blending function

Signed-off-by: Donny Yang <work@kota.moe>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>