Allow multiple pixel shaders to be specified

Current configuration allows the user to specify a single pixel shader. It'd be useful if the parameter could be a list of shaders to be applied in order, for instance, distorting the screen to make it match a CRT tube (could be be even specified as two shaders, one for horizontal curvature that, if applied alone, would simulate a Trinitron tube, and other for the vertical curvature) and another to simulate raster artifacts.

Proposed technical implementation details (optional)

I'd keep the current syntax that accepts a single string for single shaders, and also accept a list of strings for multiple shaders to be applied in order.