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Unify AspectRatio, clip, and Viewport operations

Loosely quoting some relevant parts of the spec:

  • All elements that establish a new viewport (svg, symbol, image, marker, pattern) elements have attribute viewBox. Its value is x y width height, which define a new coordinate system inside a rectangle in the parent coordinate system.
  • The rectangle in the parent coordinate system is specified by the x, y, width, height attributes, for example, in the image element or in the use element that references a symbol.
  • Positioning/scaling of the viewport is controlled by the preserveAspectRatio attribute.
  • Whether there's a clipping rectangle set on the x/y/width/height viewport or the viewBox is controlled by the clip property, which we currently don't support (#272).

#92 (closed) is about a real bug where we don't clip symbol and svg appropriately. I am not sure if this is just due to incorrect defaults on the overflow property for the symbol element, or if we are calling draw_in_viewport() incorrectly for it.

We do the scaling/positioning of the preserveAspectRatio value in aspect_ratio.rs. However, we make it the caller's responsibility to:

  • deal with viewbox.x and viewbox.y; that logic only deals with width and height.
  • deal with a non-exixtent viewBox attribute.
  • deal with clipping rectangles

It would be nice to unify all of this, including making image.rs use the viewport logic.

viewport.rs already deals with a bunch of this stuff. It doesn't handle the special case of the marker element; it would be nice to have it there, too. Note that marker does not have x, y, width, height attributes; instead it uses refX/refY for the anchor point plus a viewport size defined by markerWidth/markerHeight after scaling for markerUnits.