Add postprocessing filters

```rust
trait Program {
/// Returns postprocessing filters to be applied after rendering
/// filters may declare a texture called "input", which for the first
/// filter will contain the output of the main fragment shader, and
/// for the filters thereafter will contain the output of the previous
/// filter. The last filter in the chain will write to the display
fn filters(&self) -> &[&str] {
[]
}
}