Level does not work for some image

[aplhk]

The algorithm will skip processing when the factor is zero

image-js/src/image/filter/level.js

Line 91 in 675bafe

if (factor[j] !== 0) {

Which seems to support the case when the image was untouched

image-js/src/image/filter/level.js

Lines 72 to 73 in 675bafe

	if (min[i] === 0 && max[i] === image.maxValue) {
	factor[i] = 0;

However the case that max == min is ignored and returns untouched image, which should return a black image instead

image-js/src/image/filter/level.js

Lines 74 to 79 in 675bafe

	} else if (max[i] === min[i]) {
	factor[i] = 0;
	} else {
	factor[i] = (image.maxValue + 1 - delta) / (max[i] - min[i]);
	}
	min[i] += (0.5 - delta / 2) / factor[i];