Oblique camera projection plane clipping

[RisingThumb]

This will prevent things under the mirror/water being reflected as currently happens(currently the fix is to just not have geometry under it, but this doesn't work if you have heightmapped terrain).

This is blocked until godotengine/godot#89140 is merged. Once merged it is a case of adding the following lines to the PlanarReflection.gd script in the ready function

	reflect_camera.use_oblique_frustum = true
	reflect_camera.oblique_position = self.global_position
        reflect_camera.oblique_normal = Vector3.UP

The normal code will need to be got from the rotation, but we can look at that when the PR above is merged.