(Halide) fix so that darkenBy never goes below 0
(Halide) fix so that darkenBy never goes below 0

robstewart57/END-dsl-implementations,robstewart57/small-image-processing-dsl-implementations,robstewart57/small-image-processing-dsl-implementations,robstewart57/END-dsl-implementations,robstewart57/small-image-processing-dsl-implementations,robstewart57/END-dsl-implementations