Replace counters in AVM2 in preparation for the counter API in the timeline.
Merge pull request #1397 from tschneidereit/nat

Optimizations, mostly SWF parsing related
Merge branch 'nat' of https://github.com/mozilla/shumway into nat
Major text update (includes support for static text)
Serialize symbol IDs for debugging purposes.
Fix naming and set dirty flag once the image is loaded.
Add an option to specify how many symbols should be played.
Stub for counter API
Port Vector3D and add tests.
Merge.
Add test cases for Vector3D and Matrix3D.
Mock set `matrix3D`
Mask sourceId, a better solution is needed in the future.
Don't throw errors if arguments are not found, this was breaking AVM2 unit tests.
Prevent recursions when setting or getting the length property.
Fix ridiculously silly bug in ensure vector capacity.
Fix type-o.
Rename Timeline.Frame to Timeline.FrameDelta

... which actually describes what the class does.
Store start offset in Scene objects
Cleanups in Scene class