Additionally to sample merge and active layer, now we can only use the
layer above or below the active layer as line art source.

The line art fill is meant to work on drawing lines. Though sample merge
still is ok in many cases, the more you fill with colors, the more the
line art computation becomes unecessarily complex. Also when you use a
lot of layers with some of them already filled with colors, it makes it
impossible to colorize some line art zones with the tool. Moreover you
just don't want to have to hide every layers out there to colorize one
layer (especially background layers and such as you may want to see the
result with your background).
Thus we want to be able to set the source as a unique layer, while it
not being necessarily the active one (because you want lines and colors
on different layers). In this case, I am assuming that the color and the
line layers are next to each other (most common organization).

(cherry picked from commit c71b4916)