Get mipmap offsets for WAL header, based on mipmap sizes and start offset.
get.mipmap.data.offsets(mm0_width, mm0_height, start_at = 100L)
integer vector of length 4, the offsets.
integer, width of mipmap 0
integer, height of mipmap 0
integer, the offset at which the data starts in the file. Must be 100L for WAL format.