Provide a way to create a mesh with a maximum triangle size

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davenquinn/cesium-martini#2 reports that Martini-produced meshes covering a significant angular radius appear faceted due to the low number of triangles. This is a major problem at low zoom levels in Cesium.

Right now, I am simply falling back to a height field for low-zoom tiles, but this will not show any topography if, for instance, vertical exaggeration is enabled. It would be preferable if this could be handled in the initial mesh-generation step — would it be possible for Martini to support a "max-triangle-size" optional parameter in `getMesh` that would force continued iteration until the legs of all generated triangles were shorter?

I could accomplish this with another loop, but it seems overly complex.

Start by reading the getMesh entry point and the RTIN mesh-generation flow. Determine how an optional maximum triangle-size constraint would affect iteration, then validate that generated meshes keep triangle legs below the requested bound while preserving topography at low zoom levels.

Written by the indexing model from the issue text.