Make 'fast_composite_scaled_nearest_*' less suspicious

Make 'fast_composite_scaled_nearest_*' less suspicious

Taking address of a variable and then using it as an array looks suspicious
to static code analyzers. So change it into an array with 1 element to make
them happy. Both old and new variants of this code are correct because 'vx'
and 'unit_x' arguments are set to 0 and it means that the called scanline
function can only access a single element of 'zero' buffer.