Remove uses of .str(), .c_str() and some other methods of SGPath.
Pass SGPath directly where possible, or explicitly convert to the
appropriate 8-bit encoding.
default of 10 if given.
This improves performance _a lot_ if massively probing across
a bucket border as it prevents constant terrain unloading/reloading.
Adapt to corresponding SG change to make SGMaterialLib be ref-counted,
and have the 'reload-materials' command notify the tile-manager of this,
so it can update the options struct it passes to new tiles.
