Retrieve raw data for given mipmap level from WAL instance.
get.wal.mipmap.data(wal, mip_level)
a WAL image instance, as returned by read.wal
.
integer in range 0..3, the mipmap to plot. Level 0 is the original full-size image, the other ones get smaller and smaller (by factor 2 on each dimension, so 1/4th the size of their predecessor).