remove merge
lavfi/pad: do merge.

Notes:
 - d7dcd96a was reverted in the process, not sure if still relevant.
 - instead of FF_ARRAY_ELEMS(frame->buf), 4 is hardcoded (because draw
   only has 4 planes, and thus you get some "array subscript is above
   array bounds" warnings.
 - compilation work, output untested