Refactor edge rendering

[vitvakatu]

Our current implementation was copied line-by-line from a WebGL-based Rust implementation more than a year ago, and it is quite complicated to support. The main reason is that we were forced to introduce some performance-related constraints that are no longer relevant to us (like the need to represent each edge as a combination of basic “Rect” shapes with rounded corners).

The new implementation should be much simpler to support and reason about, and it could potentially highlight some low-hanging optimization tricks.