Fix +[CCFileUtils getDoubleResolutionImage:] for Retina

When an image and its high-resolution counterpart have different
extensions, this method would not generate a proper abslute path to the
high-resolution image.

We fix this by using an additional call to -[NSBundle
pathForResource:ofType:] when the final path appears to be relative.

cocos2d/Support/CCFileUtils.m

@@ -130,6 +130,9 @@ +(NSString*) getDoubleResolutionImage:(NSString*)path
 		NSString *retinaName = [pathWithoutExtension stringByAppendingString:CC_RETINA_DISPLAY_FILENAME_SUFFIX];
 		retinaName = [retinaName stringByAppendingPathExtension:extension];
 
+        if (![retinaName isAbsolutePath])
+            retinaName = [[NSBundle mainBundle] pathForResource:retinaName ofType:nil];
+
 		if( [__localFileManager fileExistsAtPath:retinaName] )
 			return retinaName;