Remove redundant chunk-coords passed into generate method

You can access the base chunk coordinates in the buffer. This prevents people from confusing the generator for generating a single chunk at once, instead of multiple chunks like it does.

It's better to let people access the base point information in the buffer instead of passing it into the method, since the current buffer might be re-used (performance?) at some point in time. It is better to catch it right then than to find out that one method, that was using these coordinates (such as setHorizontalLayer) suddenly fails, leaving us clueless.

Plus, it reduces how complicated the generate method looks like.

Initially I also wanted to include a World and turn the top/base into a Point, but the cuboid buffer is meant to be used without a World (offline), so I decided not to take that route.

Note: requires vanilla changes as well, see https://github.com/Vanilla/Vanilla/pull/639

Signed-off-by: Irmo van den Berge <bergerkiller@gmail.com>