Turn off threading for rasterizing really large rasters

[rafaqz]

Because we need to make separate rasters for each thread to write to, threading can use all RAM when a single thread would still work.

We should look at how to check available memory and raster size and use some heuristic.

See #529

[alex-s-gardner]

One option would be to thread by tiles, cropping the shapes to each tile would also greatly reduce the search space... though I suspect this would be a more involved mod.

[rafaqz]

Cropping the shapes may take longer than rasterizing them unfortunately.

And if there are just a few big polygons then the tiles are broken.

I suspect the key problem is actually that I switched the burning arrays from BitArray to Array{Bool} and that means 8x more memory. Maybe we can just swap that back and keep the threading in most cases... it only doubles the memory use for UInt8 arrays and 8 cores.