refactor(animation): use built-in float.Lerp/double.Lerp in transitions (#22043)

Replace hand-written linear interpolation formulas with the framework
Lerp helpers across animation code and the composition expression FFI.

samples/ControlCatalog/Pages/TabbedPage/FluidNavBar/FluidNavBar.cs

@@ -604,7 +604,7 @@ namespace ControlCatalog.Pages
                 while (measure.NextContour());
             }
 
-            private static float Lerp(float a, float b, float t) => a + (b - a) * t;
+            private static float Lerp(float a, float b, float t) => float.Lerp(a, b, t);
 
             private static double LinearPoint(double x, double pIn, double pOut)
             {

samples/ControlCatalog/Pages/Transitions/CardStackPageTransition.cs

@@ -424,10 +424,8 @@ public class CardStackPageTransition : PageSlide
         return sign * FarPeekAngle;
     }
 
-    private static double Lerp(double from, double to, double t)
-    {
-        return from + ((to - from) * Math.Clamp(t, 0.0, 1.0));
-    }
+    private static double Lerp(double from, double to, double t) =>
+        double.Lerp(from, to, Math.Clamp(t, 0.0, 1.0));
 
     private static int GetViewportZIndex(double offsetFromCenter, Visual visual, Visual? from, Visual? to)
     {

src/Avalonia.Base/Animation/CrossFade.cs

@@ -260,9 +260,7 @@ namespace Avalonia.Animation
             return FarPeekOpacity;
         }
 
-        private static double Lerp(double from, double to, double t)
-        {
-            return from + ((to - from) * Math.Clamp(t, 0.0, 1.0));
-        }
+        private static double Lerp(double from, double to, double t) =>
+            double.Lerp(from, to, Math.Clamp(t, 0.0, 1.0));
     }
 }

src/Avalonia.Base/Animation/Transitions/Rotate3DTransition.cs

@@ -244,10 +244,8 @@ public class Rotate3DTransition : PageSlide
         return 1;
     }
 
-    private static double Lerp(double from, double to, double t)
-    {
-        return from + ((to - from) * Math.Clamp(t, 0.0, 1.0));
-    }
+    private static double Lerp(double from, double to, double t) =>
+        double.Lerp(from, to, Math.Clamp(t, 0.0, 1.0));
 
     /// <inheritdoc/>
     public override void Reset(Visual visual)

src/Avalonia.Base/Rendering/Composition/Animations/Interpolators.cs

@@ -77,7 +77,7 @@ namespace Avalonia.Rendering.Composition.Animations
     
     class ColorInterpolator : IInterpolator<Avalonia.Media.Color>
     {
-        static byte Lerp(float a, float b, float p) => (byte) Math.Max(0, Math.Min(255, (p * (b - a) + a)));
+        static byte Lerp(float a, float b, float p) => (byte)Math.Clamp(float.Lerp(a, b, p), 0, 255);
 
         public static Avalonia.Media.Color
             LerpRGB(Avalonia.Media.Color to, Avalonia.Media.Color from, float progress) =>

src/Avalonia.Base/Rendering/Composition/Expressions/BuiltInExpressionFfi.cs

@@ -13,8 +13,8 @@ namespace Avalonia.Rendering.Composition.Expressions
     {
         private readonly DelegateExpressionFfi _registry;
 
-        static float Lerp(float a, float b, float p) => p * (b - a) + a;
-        static double Lerp(double a, double b, double p) => p * (b - a) + a;
+        static float Lerp(float a, float b, float p) => float.Lerp(a, b, p);
+        static double Lerp(double a, double b, double p) => double.Lerp(a, b, p);
 
         static Matrix3x2 Inverse(Matrix3x2 m)
         {