Stricter filename checking on region files, bugfixes

Fixed bug finding world dimensions

Removed some old unused variable declarations

sscanf was still matching .mcr files, but we currently only support .mca
files. This was causing parsing errors. There's room to add support for
.mcr files later on though, so I left the
check for the extension in the sscanf format string.
Load colours once and pass a pointer to the colour data

Instead of passing the filename and loading them for every chunk
rendered.
Add height shading