Move logic of computing Blurhash components to ImageProcessor

Also rename last few instances of GetImageHash to GetImageBlurHash for clarity

Emby.Drawing/NullImageEncoder.cs

@@ -44,7 +44,7 @@ namespace Emby.Drawing
         }
 
         /// <inheritdoc />
-        public string GetImageHash(string path)
+        public string GetImageBlurHash(int xComp, int yComp, string path)
         {
             throw new NotImplementedException();
         }