Set default background color to black

This ensures that the background is always opaque.

This is a significant change; before, if no background color was
specified, images were drawn onto an initially transparent buffer,
leaving some of the content behind swaybg visible if the image was
alpha transparent or if the 'fit' or 'center' scaling modes were used
and the image aspect ratio did not match, or the image was not large
enough. As the purpose of a wallpaper/background program is to draw
the pixels behind all other interface items, this transparency is
neither a required nor a very useful behavior.

Sway version >= 1.9 by default clears frames with black, so this
change should have no visible impact. Users of other compositors
may need to adjust their images or specify the --color flag.

0 files changed, 0 insertions, 0 deletions