Cache local images into memory

This introduces a cache for local image on macOS. There are no predefined limits set here (at least yet) since presumably we are loading a finite number of resources and we have the benefit of virtual memory swap on desktop so we don't really need to sweat a few extra megabytes of in-memory cache for images we know we'll need to repeatedly load.

Local images don't use the shared RN image cache, and on macOS there's no automatic in-memory cache for local resources (other than the OS-level disk cache) so the `RCTImageFromLocalBundleAssetURL` function will synchronously access the disk on the main thread many times over during thread switching in Messenger Desktop.

Added logging to confirmed cache works as expected in rn-tester, and instrumentation also confirms it.

React/Base/RCTUtils.m

@@ -21,6 +21,8 @@
 #import "RCTAssert.h"
 #import "RCTLog.h"
 
+static const NSUInteger RCTMaxCachableImageCount = 100;
+
 NSString *const RCTErrorUnspecified = @"EUNSPECIFIED";
 
 // Returns the Path of Home directory
@@ -829,11 +831,29 @@ BOOL RCTIsLocalAssetURL(NSURL *__nullable imageURL)
   return RCTImageFromLocalAssetURL(bundleImageUrl);
 }
 
+#if TARGET_OS_OSX // [TODO(macOS GH#774)
+static NSCache<NSURL *, UIImage *> *RCTLocalImageCache()
+{
+  static NSCache *imageCache;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    imageCache = [NSCache new];
+    imageCache.countLimit = RCTMaxCachableImageCount;
+  });
+  return imageCache;
+}
+#endif // ]TODO(macOS GH#774)
+
 UIImage *__nullable RCTImageFromLocalAssetURL(NSURL *imageURL)
 {
   NSString *imageName = RCTBundlePathForURL(imageURL);
 #if TARGET_OS_OSX // [TODO(macOS GH#774)
   NSURL *bundleImageURL = nil;
+
+  UIImage *cachedImage = [RCTLocalImageCache() objectForKey:imageURL];
+  if (cachedImage) {
+    return cachedImage;
+  }
 #endif // ]TODO(macOS GH#774)
 
   NSBundle *bundle = nil;
@@ -911,6 +931,13 @@ BOOL RCTIsLocalAssetURL(NSURL *__nullable imageURL)
       }
     }
   }
+
+#if TARGET_OS_OSX // [TODO(macOS GH#774)
+  if (image) {
+    [RCTLocalImageCache() setObject:image forKey:imageURL];
+  }
+#endif // ]TODO(macOS GH#774)
+
   return image;
 }