stim: remove TimeVarying3

`TimeVarying`(1) is enough for what we want.

crates/applications/wavefront/src/main.rs

@@ -11,7 +11,7 @@ use coremem::{mat, Driver};
 use coremem::geom::{Coord as _, Cube, Index};
 use coremem::units::Seconds;
 use coremem::sim::spirv::{self, SpirvSim};
-use coremem::stim::{RegionGated, TimeVarying as _, UniformStimulus};
+use coremem::stim::{RegionGated, StimExt as _, UniformStimulus};
 use coremem::cross::vec::Vec3;
 
 type Mat = mat::FullyGenericMaterial<f32>;