Demo videos show at full size as soon as a card opens

When you open a card with a demo video that has no poster image, the video now shows at its full size with its first frame. Before, it appeared as a small, flat black strip and only grew to its real size once you pressed Play. That happened because the browser sizes a video that hasn't played yet by its poster, and these videos didn't have one. The player now takes the video's proportions from the file itself, so the video keeps the right size even when a poster is missing or fails to load. Videos that have a poster look the same as before.