Without an explicit height defined on the container, Safari
fails to expand the `.picture` content. On desktop this results
in the image showing too small and at the top of the screen.
On mobile the picture ends up with zero height and is completely
hidden.
This commit fixes the issue by defining a height of 100% on the
`.lightbox` border box.
Fixes: #278
Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>