Remove default argument from MovieClip#_gotoFrame

All callsites can pass in proper values for the `scene` argument easily.
Change AVM1's ifFrameLoaded function to not return false forever if the requested frame is > totalFrames
Merge pull request #1475 from tschneidereit/nat

Change AVM1's ifFrameLoaded function to not return false forever if the requested frame is > totalFrames