Better gradient support

[Keavon]

• Gradients within strokes: SVG supports this, we just need to make the renderer and tooling use it.
• Elliptical gradients (minor and major axis)
• Focal point
• Spread method (pad, reflect, repeat)
• Gradient stop midpoint (for non-linear interpolation, using that as an "exponent" of sorts)
• Color interpolation (color-interpolation has limited browser support), we can also approximate in steps
• Independent interpolation of color and alpha
• Reusing vector data types to provide the spatial gradient control inputs