Set base altitude and crop shading range using command line parameters.

[Protected]

This allows people to tweak shading for compatibility with high altitude range worlds.

Examples:

• Old version: http://www.discworld.eu/m_e/map_example_old.jpg
• New version (with -b 160 -min-shading -3): http://www.discworld.eu/m_e/map_example_new.jpg

[TOGoS]

Sorry that took so long. I merged this but changed how it worked a little:

• I changed the names of the options to be more descriptive.
• The shading limitations only apply to height-based shading, so surface-angle shading will still take place at the upper and lower limits.
• You can change the amount of shading per change in altitude.