Port to PDN 5.1 and BitmapEffect #62
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`bmpCommitted` is mostly used in methods that operate on `S.D.Bitmap`. However, the new effect system in PDN 5.0+ operates on `IBitmap`s. In order to safely interoperate between these, without having expensive copies, the bitmap must be allocated as an `IBitmap` with an aliased `S.D.Bitmap`. This ensures that lifetime can be controlled by the `IBitmap`, which supports reference counting (via `CreateRef()`/`TryCreateRef()`). `S.D.Bitmap` has no way to link itself to another object, so the constructor overload that takes a pointer to bitmap data is very prone to use-after-free errors. The `IBitmap` implements a "link" from the `S.D.Bitmap` to itself via `ConditionalWeakTable`, which ensures that the `IBitmap` won't be garbage collected if the only reference to it is through the "link" from `S.D.Bitmap`. (so you could create the `InteropBitmap`, then use `AsGdipBitmap()`, and then forget/drop the reference to the `InteropBitmap`, and it still wouldn't get freed until the `S.D.Bitmap` is garbage collected.) Also, PDN does not provide a built-in way to create a "shared" `IBitmap` that points to memory allocated elsewhere (for good reason -- it immediately causes major bug hazards that I did see tripped over during PDN 5.0's private alpha period!). So this custom mechanism was required.
This was never supposed to have any public APIs in it, and anything left in there is now `internal` or `[Obsolete]`
Still a few things left that aren't compiling
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