Generalized graphical data rendering representation

This project refactors Graphite to create a more generalized graphic representation of paints. It introduces dedicated Gradient and Pattern nodes, as well as updated Fill and Stroke nodes that can consume generalized graphic sources, not just the current fill and color types. The project also enables the standalone rendering of colors, gradients, patterns, and other compatible graphics as ordinary layers. The implementation will update Graphite's graphics, nodes, and Vello/SVG renderers to support gradient strokes and pattern fills. Additionally, SVG import will be supported for the MVP subset of these features. Stretch goals include broader gradient support, such as focal points, conic gradients, and gradients along strokes.