There is a race condition inadvertently caused by allowing movies to be fetched in slot 0: it is possible for the player to be caught mid-load without a root movie. A lot of code assumes level 0 always exists (e.g. `levels.get(0).unwrap()`), while our initialization methods assumed no Player methods would be called until the root movie is installed. This is an unreasonable assumption, as among other things users can trigger the race condition by just playing the movie too quickly.
During the small period of time when a player is created but has no root movie, a temporary empty movie is installed with an assumed stage size and framerate of 550x400@12fps. This is Flash default for new projects, so it seemed appropriate. User ActionScript cannot see these values, and I'm not even sure JavaScript can, either.
This also results in a far reduced role for `ReturnValue`, since I also took the liberty of removing most of it's use. Furthermore, I also made it apply equally to native and AVM2 code, which ensures all native implementations of methods don't double-borrow.
In AVM1, `ReturnValue` was actually removed entirely, because it's not needed. I attempted to do the same, but the fact that we're currently embedding `ScriptObjectData` in native objects means that we need it for virtual properties. Otherwise, virtual property implementations will see locked objects, which is bad.
On the desktop player, shared objects will now be flushed on quit.
Attempting to retrieve an existing shared object will now return a
reference to the existing one.
These tests depend on the particulars of our default device font, Noto Sans. If this test proves to be fragile we may need to create a testing font with a locked width and kerning table...
If a class is registered to a clip that is placed on the timeline
during a goto, that constructor should run after the frame is
completely constructed. In order to tell whether to run the
constructor immediately, add a parameter to `post_instantiation`
to indicate if the clip is instantiated from the AVM or via a
standard timeline seek.
David Wendt