transitPathLayer: Ensure contrasting outline color
Safe-guard with a contrasting outline color. This corresponds with how it's done for the boarding markers and avoids using the default color in cases where there's only a route color, but no text color defined. As otherwise the default text color would be chosen in cases where the color is low contrast relative to the backgroud (light in light mode, or dark in dark mode).
Fixes #745 (closed)