Anchors have a memory leak

[austinbhale]

By the time an anchor is created, it already has 3 references. assets_allocate +1 anchor_addref +1 anchor_mark_dirty +1, -1 (this one's good since it gets released at the end of every step)

then for MSFT anchors, an additional: anchor_addref +1

Whenever the garbage collector in C# calls assets_releaseref_threadsafe, it decrements the reference count. For MSFT anchors this goes from 3 -> 2 and for regular anchors 2 -> 1. It persists in memory in the lists and never receives a call to its anchor_destroy method since the reference count never gets to 0. Lmk if you'd like any more debugging info, thanks!