Rework how transitions are handled in the theme.

Rework how transitions are handled in the theme.

Before, zoom and fade were too distinct; zoom was done with
live_signal always being SBS and a “zoom_poi” variable to
tell where we were zooming, while fade had its own fake signal
type. Make en explicit notion of transitions with to/from signals
instead (when transitions are in play, live_signal is just ignored,
at least for most purposes).

Again, comes from the Solskogen theme.