Fix alignment and locking issues with CommandQueueMT

The allocations of commands in CommandQueueMT weren't aligned. This
commit aligns all accesses on 64bit boundaries regardless of target
platform. This ensures that all types are aligned.

Lock-wise the semaphores were maked as usable when the command had ran
but not when the synchronous stub had finished with it. This lead to a
race condition where sometimes the semaphore got reused before it was
waited on. We now mark the semaphore as free only once we're done
waiting on it.

core/command_queue_mt.cpp

@@ -97,7 +97,7 @@ tryagain:
 		return false;
 	}
 
-	dealloc_ptr += (size >> 1) + sizeof(uint32_t);
+	dealloc_ptr += (size >> 1) + 8;
 	return true;
 }
 
@@ -107,6 +107,7 @@ CommandQueueMT::CommandQueueMT(bool p_sync) {
 	write_ptr = 0;
 	dealloc_ptr = 0;
 	mutex = Mutex::create();
+	command_mem = (uint8_t *)memalloc(COMMAND_MEM_SIZE);
 
 	for (int i = 0; i < SYNC_SEMAPHORES; i++) {
 
@@ -128,4 +129,5 @@ CommandQueueMT::~CommandQueueMT() {
 
 		memdelete(sync_sems[i].sem);
 	}
+	memfree(command_mem);
 }