Support for SVG filter effects

SVG has support for many filter effects (elements prefixed with `fe`). Documentation is available on [MDN list of SVG elements](https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Element/feDropShadow). A filter can be made by composing one or more of these filter effects. For example a glow could be made by combining the [`feMorphology`](https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Element/feMorphology) to make the element thicker and then the [`feGaussianBlur`](https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Element/feGaussianBlur) to blur the result. This can allow for powerful graphic effects whilst remaining a vector (useful for exporting and performance). Examples from the Inkscape filter gallery:

Filters can also be composed by hand using a filter editor view:

This style of approach lends itself well the the existing graphite node graph.

Currently there is implemented a raster based blur and the ability to change the blend mode (this is implemented with the blend mode not a filter effect). However this significantly limits the possibilities compared to Inkscape.

The filter effect must also be implemented in the vello rendering pipeline, attempting to match the SVG rendering in a browser. For efficient SVG output, it should be possible to combine filter effects applied to an element into a single filter that can be reused. However this is not a particular priority given the rest of the SVG output is horrible.

先研究现有的基于光栅的模糊和混合模式实现，然后追踪滤镜效果将如何通过 Graphite 节点图和 vello 渲染管线。使用链接的 MDN SVG 元素参考，将预期行为与浏览器的 SVG 渲染进行比较；完成的要求是实现组合的 SVG 滤镜效果、与 vello 输出匹配，并在可行时提供可复用的组合滤镜。

由索引模型根据 Issue 内容生成。