Add lerp function.

src/headers/vector2d.h

@@ -131,4 +131,6 @@ void frect_to_rect(const frect_t *frect, rect_t *rect);
 */
 #define intersect_frect(A, B, result) SDL_IntersectFRect(A, B, result)
 
+float lerp(float a, float b, float t);
+
 #endif // VECTOR2D_H

src/vector2d.c

@@ -15,3 +15,8 @@ void frect_to_rect(const frect_t *frect, rect_t *rect)
     rect->w = (int)frect->w;
     rect->h = (int)frect->h;
 }
+
+float lerp(float a, float b, float t)
+{
+    return a * (1 - t) + b * t;
+}