Add .buffer() to Path and FlattenedPath

[abey79]

This PR does the following:

• Remove geo feature flag from vsvg - geo is now a required dependency
• Add Path::to_geo_polygon(tolerance) to convert BezPath-based paths to geo::Polygon
• Add Polyline::to_geo_polygon() to convert polylines to geo::Polygon
• Add Path::buffer(distance, tolerance) and Polyline::buffer(distance) for polygon expand/shrink operations
• Add ToGeoPolygonError enum with variants: ExteriorNotClosed, ExteriorTooFewPoints, InteriorNotClosed(usize), InteriorTooFewPoints(usize), EmptyPath, InvalidPolygon

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This is part 2 of 4 in a stack made with GitButler:

•  4  Add support for layer/path metadata inheritance #187
•  3  Add hatching algorithm to Path and FlattenedPath #186
•  2  Add .buffer() to Path and FlattenedPath #184 👈
•  1  Add low-level implementation for path joining and merging optimization #183