Keep gradient stops closer than one LUT step on the CPU. (#4712)

The GPU ramp is 4096 entries, so stops at 0.4999 and 0.5 interpolated to gray while the CPU kept the step. Stops that share an entry now use the CPU composite. Fixes #3053.

Co-authored-by: Brandon Thomas <[email protected]>

crates/gpu/src/plan.rs

@@ -1739,7 +1739,8 @@ fn gradient_rows(g: &Gradient) -> Vec<[f32; 4096]> {
 fn check_gradient_stops<T>(stops: &[(f32, T)]) -> Result<(), Unsupported> {
     let mut positions: Vec<f32> = stops.iter().map(|s| s.0).collect();
     positions.sort_by(f32::total_cmp);
-    if positions.windows(2).any(|w| w[0] == w[1]) {
+    // A 4096-entry LUT cannot keep two stops that fall in the same entry apart (#3053).
+    if positions.windows(2).any(|w| (w[1] - w[0]).abs() < 1.0 / 4095.0) {
         return Err(Unsupported("coincident gradient stops (composited on the CPU)".into()));
     }
     Ok(())
@@ -1791,6 +1792,12 @@ mod tests {
     use photocraft_color::{Color, ColorMode, SampleType};
     use photocraft_geom::Size;
 
+    #[test]
+    fn stops_closer_than_one_lut_step_stay_on_the_cpu() {
+        assert!(check_gradient_stops(&[(0.4999_f32, ()), (0.5, ())]).is_err());
+        assert!(check_gradient_stops(&[(0.0_f32, ()), (1.0, ())]).is_ok());
+    }
+
     #[test]
     fn lab_channel_adjustments_defer_to_the_cpu_in_lab_documents() {
         let lab_rules = [