Move the default animated image runloop mode to a constant, and fallback to the constant if the runloop mode is set to nil

AsyncDisplayKit/ASImageNode+AnimatedImage.mm

@@ -18,6 +18,8 @@
 #import "ASInternalHelpers.h"
 #import "ASWeakProxy.h"
 
+NSString *const ASAnimatedImageDefaultRunLoopMode = NSDefaultRunLoopMode;
+
 @implementation ASImageNode (AnimatedImage)
 
 #pragma mark - GIF support
@@ -95,6 +97,10 @@
 {
   ASDN::MutexLocker l(_displayLinkLock);
 
+  if (runLoopMode == nil) {
+    runLoopMode = ASAnimatedImageDefaultRunLoopMode;
+  }
+
   if (_displayLink != nil) {
     [_displayLink removeFromRunLoop:[NSRunLoop mainRunLoop] forMode:_animatedImageRunLoopMode];
     [_displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:runLoopMode];

AsyncDisplayKit/ASImageNode.mm

@@ -105,7 +105,7 @@
   _cropRect = CGRectMake(0.5, 0.5, 0, 0);
   _cropDisplayBounds = CGRectNull;
   _placeholderColor = ASDisplayNodeDefaultPlaceholderColor();
-  _animatedImageRunLoopMode = NSDefaultRunLoopMode;
+  _animatedImageRunLoopMode = ASAnimatedImageDefaultRunLoopMode;
   
   return self;
 }

AsyncDisplayKit/Private/ASImageNode+AnimatedImagePrivate.h

@@ -8,6 +8,8 @@
 
 #import "ASThread.h"
 
+extern NSString *const ASAnimatedImageDefaultRunLoopMode;
+
 @interface ASImageNode ()
 {
   ASDN::RecursiveMutex _animatedImageLock;