fixed ByteBuffer management

this should be (.position 0) to ensure the buffer is at the beginning

src/play_clj/core_utils.clj

@@ -21,7 +21,7 @@ specified path.
       (doto pixels
         (.clear)
         (.put pixel-data)
-        (.position))
+        (.position 0))
       pic))
   ([path]
     (let [pic (screenshot!)