Add needsPrescaling public property

- This exposes the ability to turn off frame prescaling.
- Closes #40

Source/AnimatableImageView.swift

@@ -10,6 +10,9 @@ public class AnimatableImageView: UIImageView {
   /// The size of the frame cache.
   public var framePreloadCount = 50
 
+  /// Specifies whether the GIF frames should be pre-scaled to save memory. Default is **true**.
+  public var needsPrescaling = true
+  
   /// A computed property that returns whether the image view is animating.
   public var isAnimatingGIF: Bool {
     return !displayLink.paused
@@ -35,6 +38,7 @@ public class AnimatableImageView: UIImageView {
   public func prepareForAnimation(imageData data: NSData) {
     image = UIImage(data: data)
     animator = Animator(data: data, size: frame.size, contentMode: contentMode, framePreloadCount: framePreloadCount)
+    animator?.needsPrescaling = needsPrescaling
     animator?.prepareFrames()
     attachDisplayLink()
   }

Source/Animator.swift

@@ -23,6 +23,9 @@ class Animator {
   var currentPreloadIndex = 0
   /// Time elapsed since the last frame change. Used to determine when the frame should be updated.
   var timeSinceLastFrameChange: NSTimeInterval = 0.0
+  /// Specifies whether GIF frames should be pre-scaled.
+  /// - seealso: `needsPrescaling` in AnimatableImageView.
+  var needsPrescaling = true
 
   /// The current image frame to show.
   var currentFrame: UIImage? {
@@ -69,11 +72,15 @@ class Animator {
     let image = UIImage(CGImage: frameImageRef)
     let scaledImage: UIImage?
 
+    if needsPrescaling == true {
       switch contentMode {
       case .ScaleAspectFit: scaledImage = image.resizeAspectFit(size)
       case .ScaleAspectFill: scaledImage = image.resizeAspectFill(size)
       default: scaledImage = image.resize(size)
       }
+    } else {
+      scaledImage = image
+    }
 
     return AnimatedFrame(image: scaledImage, duration: frameDuration)
   }