Accept float stroke widths

[radarhere]

Resolves #8359

When instructing FreeType of the stroke width, we multiply the number of pixels by 64.

Pillow/src/_imagingft.c

Lines 951 to 953 in cb2a0c4

	FT_Stroker_Set(
	stroker,
	(FT_Fixed)stroke_width * 64,

So there's no reason why stroke widths couldn't be more precise than an integer number of pixels, and instead going down to 1/64th of a pixel.

To demonstrate, here's a progression from no stroke to 1px stroke width.

from PIL import Image, ImageDraw, ImageFont

im = Image.new("RGB", (360, 120))
d = ImageDraw.Draw(im)
font = ImageFont.load_default(size=100)
d.text((5, 0), "A", font=font, fill="#f00")
d.text((75, 0), "A", font=font, fill="#f00", stroke_fill="#0f0", stroke_width=0.25)
d.text((145, 0), "A", font=font, fill="#f00", stroke_fill="#0f0", stroke_width=0.5)
d.text((215, 0), "A", font=font, fill="#f00", stroke_fill="#0f0", stroke_width=0.75)
d.text((285, 0), "A", font=font, fill="#f00", stroke_fill="#0f0", stroke_width=1)

im.save("out.png")