Currently markers are structs, but I'm unsure if this is the best strategy. It results in marker designators being decoupled from marker rendering code and makes it really hard for users to inject custom markers.

The advantage of keeping MarkerShape a struct is that it can have clear default values in argument lists, but SP5 tries to move away from methods with long argument lists, so it turns out that this isn't a strong advantage after all.

markers are confusing because some have separate outline and fill colors