Feature request: Rounded corners

[shannonturner]

A frequently-requested feature is being able to add rounded corners to lines, to have the map feel less as though it's on a grid.

While it may look sleek, it would require either a careful rework of the line drawing algorithm, or potentially removing the grid, or both.

Worth thinking carefully about the best way to handle this while balancing the maintainability of everything else; it would likely require a mapDataVersion bump.