Available services (all canvas implementations should provide
 this minimal set, though are free to add more; just check the
 getAvailableServiceNames() on the returned interface):

 - Gradients - all gradients need to support two construction
 parameters, "Colors" being a ::sequence
 and "Stops" being a ::sequence. Both must
 have the same length, and at least two elements. See
 http://www.w3.org/TR/SVG11/pservers.html#GradientStops for
 the semantics of gradient stops and colors.
 Required gradient services:

 * "LinearGradient" - the gradient varies linearly between
 the given colors. without coordinate system
 transformation, the color interpolation happens in
 increasing x direction, and is constant in y
 direction. Equivalent to svg linear gradient
 http://www.w3.org/TR/SVG11/pservers.html#LinearGradients

 * "EllipticalGradient" - this gradient has zeroth color
 index in the middle, and varies linearly between center
 and final color. The services takes an additional
 parameter named "AspectRatio" of ::double
 (width divided by height), if this aspect ratio is 1, the
 gradient is circular. If it's not 1, the gradient is
 elliptical, with the special twist that the aspect ratio
 is maintained also for the center color: the gradient will
 not collapse into a single point, but become a line of
 center color. If "AspectRatio" is missing, or equal to 1,
 this gradient yields similar results as the svg radial
 gradient
 http://www.w3.org/TR/SVG11/pservers.html#RadialGradients

 * "RectangularGradient" - this gradient has zeroth color
 index in the middle, and varies linearly between center
 and final color via rectangular boxes
 around the center point. The services takes an additional
 parameter named "AspectRatio" of ::double
 (width divided by height), if this aspect ratio is 1, the
 gradient is quadratic. If it's not 1, the gradient is
 rectangular, with the special twist that the aspect ratio
 is maintained also for the center color: the gradient will
 not collapse into a single point, but become a line of
 center color.

 - Hatch patterns - Required hatch services:

 * "VerticalLineHatch" - this hatching consists of vertical lines
 * "OrthogonalLinesHatch" - this hatching consists of
 crossing vertical and horizontal lines
 * "ThreeCrossingLinesHatch" - this hatching consists of
 vertical and horizontal lines plus diagonal lines from
 left, top to bottom, right.
 * "FourCrossingLinesHatch" - this hatching consists of
 vertical and horizontal lines plus diagonal lines in both
 directions.