It is only of interest to log the currently active layers when debugging.
A layer is inactive when it does not have an fb and composition is not forced.
Check layers for that and otherwise skip them. Also tell if composition is
actually forced on the layer.
The CRTC_X and CRTC_Y values can be negative. Log respectively. Additionally
the SRC_W and SRC_H values are shifted. Shift these values back for the log.
This lets the library user define a layer where composition will happen. At the
moment there can be at most one composition layer per output, on the primary
plane. It should be possible to remove these restrictions in the future if
desirable.
The composition layer will be put on a plane if and only if composition is
needed.
The compositor needs to know where composited layers will be blended to resolve
zpos conflicts.
Closes: https://github.com/emersion/libliftoff/issues/9
