Only release tiles when that can be useful

exercises/src/gpu/grayscott.comp

@@ -320,8 +320,8 @@ void run_simulation() {
         #endif
     }
 
-    // Liberate any acquired tile
-    if (s_acquired_tile && is_leader()) {
+    // Liberate any acquired tile that was dropped due to an early exit
+    if (is_leader() && s_acquired_tile && work_group_finished()) {
         leader_release_tile();
         #if DEBUG || PROFILE
             atomicAdd(g_metadata.final_release, 1);