Refactor ChunkyPNG::Dimension and ChunkyPNG::Point factories for improved readability.

I find long nested `case`/`when` statements difficult to read, so I pulled some of the guts for these two factory methods into clearly-named private helpers.  I also broke lines over 80 characters into multiple lines, but stuck with the project's overall style of inline `when`'s for those that were not very long.