Add a Viewport method to get automatically computed 2D stretch transform

`Viewport.get_stretch_transform()` returns the automatically computed
2D stretch transform. Combined with `Transform2D.get_scale()`, this is
useful when using the `canvas_items` stretch mode in a project.

There are many situations where knowing this factor is useful:

- Divide Camera2D zoom to keep the size of the 2D game world identical
  regardless of the 2D scale factor (so that UI elements can still be scaled).
- Make certain controls always drawn at 1:1 scale
  (e.g. for the crosshair in a FPS). This is done by dividing the Control
  node's scale by the scale factor.

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