mosaic() turns the picture into square blocks, each the colour at its middle. The consoles of the era had it in hardware, switched on from a single register, and games used it to dissolve into a room, out of a hit, or into a dream.
size is how many game pixels wide a block is, and 1 is nothing at all. It is meant to be animated: press Enter for the classic dissolve, which only writes effect.uniforms.size every frame, up to 24 and back down.
↑ ↓ to change the size by hand and Space to switch it off.