Previously all the `.as` files compiled into `playerglobal.swf`
were detected automatically using `walkdir`. While this might be
convenient, it can cause unexpected results when untracked `.as`
files exist. So instead, introduce two entry points - `stubs.as`
and `globals.as`, which include all stub ActionScript definitions,
and actual class implementations, respectively. This also simplifies
the `playerglobal.swf` build script a bit.
This PR adds support for building a custom `playerglobal.swf`, which can be used
to implement builtin Flash classes in ActionScript. This file is embedded into Ruffle
using `include_bytes!`, and loaded during initialization.
As an example, the `Point` class is reimplemented
in ActionScript, and `flash.text.AntiAliasType` is added.
The ActionScript compilation process is performed by `core/build.rs`.
See that file, along with `core/src/avm2/globals/README.md`, for
more details.
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