Corrected drawing I;16 points and writing I;16 text

[radarhere]

Resolves #7245

As the issue explains,

Pillow/src/libImaging/Draw.c

Lines 68 to 75 in be828f4

	point8(Imaging im, int x, int y, int ink) {
	if (x >= 0 && x < im->xsize && y >= 0 && y < im->ysize) {
	if (strncmp(im->mode, "I;16", 4) == 0) {
	im->image8[y][x * 2] = (UINT8)ink;
	im->image8[y][x * 2 + 1] = (UINT8)ink;
	} else {
	im->image8[y][x] = (UINT8)ink;
	}

is just drawing the writing the same value to two different points in our 8-bit image, whereas the point of 16-bit images is that each of those 2 8-bit values should be different.

This fixes that. It also means that the test from #3899 is slightly changed. The important part of that test was the width of the rectangle, which is maintained here.

The issue also requested a similar change for writing text, so I've added that commit for that.