fixes to layers managing symbols via visibility

remove frameOverShootDisplay sprite:
Having an extra _frameOvershootDisplay Sprite is unnecessary in the new visibility scheme.

fix getBounds for Movies to not include placeholder Sprites used in Layers where no symbol exists on a frame
I hadn't meant to submit this just yet but it went through on the same branch pull request, so here it is for your review.  If this is feeling heavy handed to you guys, let me know.