Added linear easing function.

src/mapping/easings.js

@@ -0,0 +1,25 @@
+
+/**
+ * @callback interpolator
+ * @param {number} current the current value.
+ * @param {number} dest the destination value.
+ * @param {number} frameDelta the time elapsed since the last update.
+ * @returns {number} the new current value.
+ */
+
+// TODO: Add some interpolation functions.
+
+/**
+ * 
+ * @param {number} rate the number of frequency values to shift by per second.
+ * @returns {interpolator} the interpolation function with the given rate.
+ */
+export function createEaseLinear(rate) {
+    return (current, dest, frameDelta) => {
+        let direction = 1;
+        if (dest < current) {
+            direction = -1;
+        }
+        return current + (Math.min(rate * frameDelta, dest) * direction);
+    };
+}