filter_kernels: fix nearest scaler

The previous commit assumed the filter would be 1x1 (then constant
weight is correct) - but our code in fact uses at least a 2x2 filter. A
1x1 filter would generally be useless, except for nearest scaling - so
it didn't exist.

Insteasd of adding such a 1x1 filter, just turn the nearest weight
function into a scare function, which should take care of the issue.