Support fill-opacity

[dirkschulze]

fill-opacity does require that we clip out the shape from the created drop shadow / outer glow. This is not really happening yet.

SVG filters do not provide clipping or masking yet. Simulating is difficult, since the alpha channel of the filtered shape doesn't seem to influence the drop shadow for instance. The area that is patched by the source graphic counts. Just like box-shadow in CSS I assume.

[jhatwich]

I think this one is fixed, please close it if you agree @dirkschulze