Do not let Xcode compressed APNG files.

Demo/YYImageDemo.xcodeproj/project.pbxproj

@@ -442,6 +442,7 @@
 				LIBRARY_SEARCH_PATHS = "";
 				PRODUCT_BUNDLE_IDENTIFIER = com.ibireme.YYImageDemo;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				STRIP_PNG_TEXT = NO;
 			};
 			name = Debug;
 		};
@@ -457,6 +458,7 @@
 				LIBRARY_SEARCH_PATHS = "";
 				PRODUCT_BUNDLE_IDENTIFIER = com.ibireme.YYImageDemo;
 				PRODUCT_NAME = "$(TARGET_NAME)";
+				STRIP_PNG_TEXT = NO;
 			};
 			name = Release;
 		};