Eagle Double Headed needs `legged` property

As of now, the charge Eagle Double Headed can only be coloured specifically on its talons, beaks, and tongues (`armed`, `beaked`, and `langued` respectively), apart from its body of course.
However, its legs cannot be coloured differently than its body, unlike with Eagle Displayed. On many coats of arms, this part of the bird is coloured differently to its body.

No file or test is named. Start by locating the Eagle Double Headed renderer and comparing it with Eagle Displayed, which already supports differently coloured legs. Done means the `legged` property can colour the legs independently while preserving the existing `armed`, `beaked`, and `langued` behaviour.

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