    paper.defineFilter(filterDefinition)

Define an SVG filter for later reuse within the paper. The method returns the filter id and the `filterDefinition` must have the following form:

    {
        name: <name of the filter>,
        args: <filter arguments>
    }

Where `name` is the name of the filter. See below for the list of built-in filters. `args` is an object containing filter parameters. These parameters are dependent on the filter used and are described in the list below as well. Example usage:

    var filterId = paper.defineFilter({
        name: 'dropShadow',
        args: {
            dx: 2,
            dy: 2,
            blur: 3
        }
    });

    svgNode.setAttribute('filter', 'url(#' + filterId + ');

The following is the list of built-in filters. All these filters are defined in the `joint.util.filter` namespace. This namespace can be extended simply by adding a new method to it with one argument, an object with filter parameters, returning a string representing the SVG filter definition.

- `blur`
  - `x` - horizontal blur
  - `y` - vertical blur [optional, if not defined `y` is the same as `x`]
- `dropShadow`
  - `dx` - horizontal shift
  - `dy` - vertical shift
  - `blur` - blur
  - `color` - color
  - `opacity` - opacity
- `grayscale`
  - `amount` - the proportion of the conversion. `1` is completely grayscale. `0` leaves the element unchanged.
- `sepia`
  - `amount` - the proportion of the conversion. `1` is completely sepia. `0` leaves the element unchanged.
- `saturate`
  - `amount` - the proportion of the conversion. `0` is completely un-saturated. `1` leaves the element unchanged.
- `hueRotate`
  - `angle` - the number of degrees around the color circle the input samples will be adjusted
- `invert`
  - `amount` - the proportion of the conversion. `1` is completely inverted. `0` leaves the element unchanged.
- `brightness`
  - `amount` - the proportion of the conversion. `0` makes the element completely black. `1` leaves the element unchanged.
- `contrast`
  - `amount` - the proportion of the conversion. `0` makes the element completely black. `1` leaves the element unchanged.