sample[LuaStudio] Prerender actions

"Change backgroung just before each render step"
FireSample example!
| y sign |
y := 128. sign := 1.
View3D preRenderActions add: [:v|
	v backgroundColor: y.
	y := y + sign.
	(y between: 5 and: 250) ifFalse: [ sign := -1 * sign ]
]!