New filter primitives

[lordmauve]

We should expand the list of filter primitives with reference to those defined for SVG.

Some of these would be immediately useful

• Color matrices, and ~the shorthand versions like grayscale, sepia~ etc.
• Dilate/erode
• Vignette

Some, like compositing operations, would need a system of filter wiring so that the output of one filter can be the input to another, and potentially so that multiple layers can be the inputs.

[lordmauve]

Chromatic aberration would also be good for retro effects.