`IImage` represents a base interface for raster and vector images. `IBitmap` now implements this interface and `DrawingContext` accepts this interface in `DrawImage`.

The interface defines a `Draw` method which introduces a level of indirection for drawing the image through the image itself.

Renamed `IDrawingContextImpl.DrawImage` to `DrawBitmap` as this only handles drawing bitmap images. `Bitmap` now calls this method directly on the platform implementation.

`Image` now bases the size of a bitmap on `IImage.Size`.

To display `DrawingImage`.

- Remove opacity parameter - should push an opacity instead
- Add a WPF-compatible overload of `DrawImage` to `DrawingContext`

And add some XML docs.

Skia doesn't support getting DPI from loaded bitmaps and always returns 96. Do the same in D2D.