DemotivatorGen: fix input picture object not being destroyed

SimpleTGBot/MemeGen/DemotivatorGen.cs

@@ -148,6 +148,7 @@ public class DemotivatorGen
             currentOrigin.Y += bt + pad;
         }
 
+        picture.Dispose();
         MemoryStream outStream = new MemoryStream();
         bitmap.Save(outStream, ImageFormat.Png);
         outStream.Seek(0, SeekOrigin.Begin);