A very silly post really, but while working on something else, I came across it and decided to take a look.
in src/_backend_agg_wrapper.cpp:
// TODO: This really shouldn’t be mutable, but Agg’s renderer buffers aren’t const.
auto image = image_obj.mutable_unchecked<2>();
I don’t really see a workaround for this because in extern/agg24-svn/include/agg_rendering_buffer.h:
memcpy(row_ptr(0, y, w), src.row_ptr(y), l);
and
*p++ = value;
ensures that the image array cannot be made const (non-mutable) as the image buffer may be modified via the copy_from and clear functions, respectively. These functions are part of the external AGG library.
So, the TODO statement seems redundant.