Add South Africa national route shields

[claysmalley]

Adds shield support for national routes of South Africa. National routes have a blue pentagonal shield with a white border and yellow text. All routes in South Africa have a letter prefix, which is shown on shields in small font above the route number, though for the purposes of this project, the prefix is displayed inline with the route number.

South Africa has additional route networks on a more local level (provincial, regional, metropolitan), though tagging would need to be more complete and consistent before these routes can have shield support: #484

[1ec5]

For context, South Africa’s directions signs follow a UK design: no shield, just a number in plain text in gold, with the alphabetic designation inline.

However, the blue pentagon remains in the SADC-RTSM and reportedly on maps. On the pentagon, the “N” appears above the route number, and a cardinal direction can optionally appear below the route number. I think it would be suboptimal to include inline prefixes on these shields or the ones in #484, just as it would be suboptimal to include “US” on a California-style U.S. Route shield.

[ZeLonewolf]

This seems like a reasonable graphical representation to me based on the limited photos shared.

[1ec5]

Between this and #162, I suspect the prefixes are just there for consistency with way refs. The photo above isn’t representative of how routes are marked on maps. Normally the pentagon contains just the route number, or it has “N” above the route number, just like on what would be the trailblazer sign.

I think we should at least give the South African community a heads-up about this issue or address it on our end via #141. If we land national route shields with the prefixes, they’ll look out of place next to provincial and metropolitan routes (#484), which likely won’t have enough horizontal space for an inline prefix.

[claysmalley]

I guess a potential solution would be to treat the prefix as banner text floating above the shield, which would involve removing the prefixes from ref values in route=road relations. I had been considering something similar with Brazil (#425).

[claysmalley]

Shield icon scrapped in favor of draw function: