Lock mutex when animation task check + completed task moving

Lock mutex when animation task check + completed task moving

Since VectorAnimationThread::OnTaskCompleted() could be called
from various worker threads, we should not change the thread's member value.

To avoid this kind of thread issue,
 - Make mutex during animation tasks control (VectorAnimationThread vs EventThread)
 - Collect tasks from worker thread and move them at Rasterize() API (VectorAnimationThread vs WorkerThread)

Change-Id: Ib3dce6b98f6c66a3bd597a7b46fe8bbac60105b5
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>