change lerp

the previous lerp implementation could cause in certain scenarios that if t = 1, the output would be different than the input.
Because of floating point precision, `a - (b - a)` wasn't guaranteed to return `b`.
Down the line, when using trim paths for example, this small difference could cause the path not to draw properly.
Changing it to this implementation solves the issue.

Diffs=
1be3488d52 change lerp (#9000)

Co-authored-by: hernan <[email protected]>

.rive_head

@@ -1 +1 @@
-19b64a2c1bbe9d0e1e7166b3290e34e3e291b018
+1be3488d52ec4c2f32a03eb111afca5b1f01ecd8

include/rive/math/math_types.hpp

@@ -150,7 +150,7 @@ RIVE_ALWAYS_INLINE static float degreesToRadians(float degrees)
 
 template <typename T> T lerp(const T& a, const T& b, float t)
 {
-    return a + (b - a) * t;
+    return a * (1 - t) + b * t;
 }
 } // namespace rive
 

tests/unit_tests/runtime/bezier_utils_test.cpp

@@ -530,9 +530,9 @@ TEST_CASE("find_cubic_convex_180_chops", "[bezier_utils]")
 TEST_CASE("find_cubic_convex_180_chops_lines", "[bezier_utils]")
 {
     Vec2D p0 = {123, 200}, p3 = {223, 432};
-    for (float t0 = 0; t0 < 1; t0 += .12f)
+    for (float t0 = 1e-3f; t0 < 1; t0 += .12f)
     {
-        for (float t1 = t0 + .097f; t0 < 1; t0 += .097f)
+        for (float t1 = t0 + .097f; t1 < 1; t1 += .097f)
         {
             Vec2D line[4] = {p0, lerp(p0, p3, t0), lerp(p0, p3, t1), p3};
             float _[2];