vp9-resize: Force reference masking off for external dynamic-resizing.
An issue exists with reference_masking in non-rd pickmode for spatial scaling. It was kept off for internal dynamic resizing and svc, this change is to keep it off also for external dynamic resizing. Update to external resize test, and update TODO to re-enable this at frame level when references have same scale as source. Change-Id: If880a643572127def703ee5b2d16fd41bdbf256c