Use make-path-stencil in \path markup command
Add two keyword arguments to make-path-stencil, allowing to configure the line cap and line join styles.
Use make-path-stencil in \path, deduplicating the code to convert paths to absolute commands and compute their extents.
As a corollary, starting paths with moveto or rmoveto is no longer necessary for paths to work in SVG output: make-path-stencil prepends moveto automatically.