fixed another error with analytic derivative

Author: jackyarndley

high.toml

@@ -22,5 +22,5 @@ valid_iteration_probe_multiplier = 0.01
 
 experimental = true  
 
-analytic_derivative = false
+analytic_derivative = true
 jitter = false

src/math/perturbation.rs

@@ -126,12 +126,12 @@ impl Perturbation {
 
                         if reference_data.extended_precision_required {
                             // If the reference is small, use the slow extended method
-                            pixel.delta_current *= reference_data.z_extended * 2.0;
-                            pixel.delta_current += pixel.delta_reference;
-
-                            pixel.derivative_current *= reference_data.z_extended * 2.0;
+                            pixel.derivative_current *= (reference_data.z_extended + pixel.delta_current) * 2.0;
                             pixel.derivative_current += ComplexExtended::new2(1.0, 0.0, 0);
 
+                            pixel.delta_current *= reference_data.z_extended * 2.0 + pixel.delta_current;
+                            pixel.delta_current += pixel.delta_reference;
+
                             // reset the scaled counter
                             pixel.delta_current.reduce();
                             pixel.derivative_current.reduce();