The separable convolution filter supports a subsample_bits of 0 which
corresponds to no subsampling at all, so allow this value to be used
in the scale demo.
For large upscalings the level of subsampling for the filter has a
quite visible effect, so make it settable in the UI so that people can
experiment with various values.
This program allows interactively scaling and rotating images with
using various filters and repeat modes. It uses
pixman_filter_create_separate_convolution() to generate the filters.
