Constant UNIFORMS

const UNIFORMS: &str = "sampler2D tex — the layer\n\
vec2 u_res — layer size in pixels\n\
float u_time — clip-local seconds\n\
float u_scale — preview scale (multiply pixel-sized amounts by it)\n\
float p0..p7 — the eight knobs, also named u1..u8\n\
sampler2D u_mask, int u_has_mask — the effect's mask (applied for you)";

What shaders::user_shader puts in scope — the knobs are invisible otherwise.