Previously, the viewport height and width were stored in
both `Stage` and the `RenderBackend`. Any changes to the viewport
dimensions (e.g. due to window resizing) needed to be updated in both
places to keep our handling of the viewport consistent.
This PR adds a new `ViewportDimensions` type, which holds the
width, height, and scale factor. It is stored inside the
`RenderBackend` impl, and is retrieved using the newly added
method `RenderBackend.get_viewport_dimensions`. After a `Player`
has been constructed, any code that needes access to the viewport
dimensions will ultimate go through this method.
Unfortunately, `Stage` needs to use the viewport dimensions
in `build_matrices`. Therefore, any code modifying the viewport
dimensions should go through `player.set_viewport_dimensions`,
which ensures that the stage matrices are rebuilt after the render
backend is updated.
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