Support for skylight shadows in isometric mode

In night mode, block light is used for shadows; in day mode, skylight.

Shadows are a little buggy due to the way some blocks store skylight
data, so they must be explicitly enabled with the -s option.
Switch to getopt for option parsing, change world path handling

- Now using getopt_long for parsing options.
- World path now expected, rather than region path. Also updated the
maximum path length, and added some error checking.
- Errors are now directed to stderr.
- Also moved the asterisk on all pointer declarations.