Correct drawing I;16 horizontal lines

[radarhere]

Currently, I;16 horizontal lines draw the same value to both 8-bit parts, rather than using the full 16 bits.

Pillow/src/libImaging/Draw.c

Lines 121 to 124 in 5a04b95

	pixelwidth = strncmp(im->mode, "I;16", 4) == 0 ? 2 : 1;
	memset(
	im->image8[y0] + x0 * pixelwidth, (UINT8)ink, (x1 - x0 + 1) * pixelwidth
	);