Use/output transparent images

[hellocatfood]

I'm trying to make Poppy work with transparent images. These are the images that I'm working with

FFMPEG is able to generate videos with an alpha channel (some details here), and with the fourcc value RGBA I ran the following command

./src/poppy -f 10 -u FFV1 -o output.mkv cat.png dog.png

ffprobe reports that the output video has an alpha channel

Input #0, matroska,webm, from 'output.mkv':
  Metadata:
    ENCODER         : Lavf60.3.100
  Duration: 00:00:00.33, start: 0.000000, bitrate: 240262 kb/s
  Stream #0:0: Video: rawvideo (RGBA / 0x41424752), rgba, 500x500, 30 fps, 30 tbr, 1k tbn
    Metadata:
      DURATION        : 00:00:00.333000000

However, when I convert it to a series of pngs the images still have a black background. Is there any way to process transparent images and have them retain their transparency?