Fix the next thread is skipped after clone deleted

Problem:
When a clone is deleted, its thread is removed from the thread
list; but the index "i" in the execution for-loop will still +1,
making the next thread skipped.

Changes:
Added a flag "isKilled" on a thread;
Set the flag when a thread is removed from the thread list;
reduce the "i" counter if the thread of the current loop is removed.

src/engine/sequencer.js

@@ -69,6 +69,9 @@ class Sequencer {
                     // Normal-mode thread: step.
                     this.stepThread(activeThread);
                     activeThread.warpTimer = null;
+                    if (activeThread.isKilled) {
+                        i--;    // if the thread is removed from the list (killed), do not increase index
+                    }
                 }
                 if (activeThread.status === Thread.STATUS_RUNNING) {
                     numActiveThreads++;