demo_fade

The last pass in the demo's post chain (priority 200). It draws the letterbox bars and the fade to a solid colour, so every part is cut, held and faded through one pass rather than per-part geometry.

bars is the height of each bar as a fraction of the frame, clamped to 0.45, and the bars are black. fade mixes the frame toward col_r/col_g/col_b, so fade = 1 is a full dip to that colour. preseed.fx drives all five through postprocess.setProperty from Fx.setFade and Fx.setBars.