Fix ever-increasing `_nextFrame` value on MovieClip instances with one frame

I guess they shouldn't ever even be calling advanceFrame, but I had an intermittendly-occurring case of this, and the check isn't incorrect in and of itself.
Work around tsc bug that causes `extends` clauses not to qualify the base class in d.ts output

`export class Region extends Rectangle` isn't turned into `class Region extends Geometry.Rectangle` even though `Geometry.Rectangle` is correctly imported. Causes error when compiling player.
Commit compiled avm1lib.abc

Needs updating after changes in 9d4da81.
Merge pull request #1436 from tschneidereit/nat

Small fixes