Resize the GIF with aspect ratio

We can use the contentMode of the UIImageView to resize the GIF keeping
the aspect ratio if the content mode is set to do so.

Source/Animatable.swift

@@ -1,6 +1,7 @@
-/// Protocol that requires its members to have a `layer` and a `frame` property.
+/// Protocol that requires its members to have a `layer`, `frame`, and `contentMode` property.
 /// Classes confirming to this protocol can serve as a delegate to `Animator`.
 protocol Animatable {
   var layer: CALayer { get }
   var frame: CGRect { get }
+  var contentMode: UIViewContentMode { get }
 }

Source/Animator.swift

@@ -57,8 +57,17 @@ class Animator: NSObject {
 
       let frameDuration = CGImageSourceGIFFrameDuration(imageSource, index)
       let frameImageRef = CGImageSourceCreateImageAtIndex(imageSource, index, nil)
-      let frame = UIImage(CGImage: frameImageRef)?.resize(size)
-      let animatedFrame = AnimatedFrame(image: frame, duration: frameDuration)
+
+      let image = UIImage(CGImage: frameImageRef)
+      let scaledImage: UIImage?
+
+      switch delegate.contentMode {
+      case .ScaleAspectFit: scaledImage = image?.resizeAspectFit(size)
+      case .ScaleAspectFill: scaledImage = image?.resizeAspectFill(size)
+      default: scaledImage = image?.resize(size)
+      }
+
+      let animatedFrame = AnimatedFrame(image: scaledImage, duration: frameDuration)
 
       return (accumulatedFrames + [animatedFrame], accumulatedDuration + frameDuration)
     }

Source/UIImageExtension.swift

@@ -50,14 +50,19 @@ extension UIImage {
 }
 
 private extension CGSize {
+  /// Calculates the aspect ratio of the size.
+  ///
+  /// :returns: aspectRatio The aspect ratio of the size.
+  var aspectRatio: CGFloat {
+    if height == 0 { return 1 }
+    return width / height
+  }
+
   /// Finds a new size constrained by a size keeping the aspect ratio.
   ///
   /// :param: size The contraining size.
   /// :returns: size A new size that fits inside the contraining size with the same aspect ratio.
   func sizeConstrainedBySize(size: CGSize) -> CGSize {
-    if height == 0 { return size }
-
-    let aspectRatio = width / height
     let aspectWidth = round(aspectRatio * size.height)
     let aspectHeight = round(size.width / aspectRatio)
 
@@ -73,9 +78,6 @@ private extension CGSize {
   /// :param: size The contraining size.
   /// :returns: size A new size that fills the contraining size keeping the same aspect ratio.
   func sizeFillingSize(size: CGSize) -> CGSize {
-    if height == 0 { return size }
-
-    let aspectRatio = width / height
     let aspectWidth = round(aspectRatio * size.height)
     let aspectHeight = round(size.width / aspectRatio)