feat: new API for extracting Image from a channel given the frame index

Author: AlmasB

fxgl-core/src/main/kotlin/com/almasb/fxgl/texture/AnimationChannel.kt

@@ -99,6 +99,15 @@ class AnimationChannel(val image: Image,
     fun getFrameWidth(frame: Int) = getFrameData(frame).width
     fun getFrameHeight(frame: Int) = getFrameData(frame).height
 
+    /**
+     * @return a computed image for a given [frameIndex]
+     */
+    fun getFrameImage(frameIndex: Int): Image {
+        val frameData = getFrameData(frameIndex)
+
+        return Texture(image).subTexture(frameData.viewport).image
+    }
+
     /**
      * Returns next frame index or 0 if [frame] is last.
      */