Tiling issues

[morishuz]

There are some issues with tile boundaries:

1. in certain circumstances border vertices are not shared with neighbouring tiles - causing discontinuities between tiles.
   

2. tiling code can produce extra vertices with spacings much less than original terrain raster (amongst other things causing severe issues when save with quantised mesh methods). To understand the scale: in this image the grey square regions are pixels!
   

please note: some of these issues may be fixed now, but either way we should have a test to measure tile boundary continuity and multiple polygons per pixel problems.

also, to visualise this i had to write specialised code (export geojson meshes in correct coordinates) but we should eventually add this as a debug option to the tiling "dem2tiles" subcommand.