Added text stroking

[radarhere]

Resolves #2209. Replacement for #2224

Adds strokeWidth and strokeFill arguments, such that -

from PIL import Image, ImageDraw, ImageFont

font = ImageFont.truetype("Tests/fonts/FreeMono.ttf", 40)
font.getsize_multiline("A", strokeWidth=2)
font.getsize("ABC\nAaaa", strokeWidth=2)

im = Image.new("RGB", (100, 100))
draw = ImageDraw.Draw(im)
draw.textsize("A", font, strokeWidth=2)
draw.multiline_textsize("ABC\nAaaa", font, strokeWidth=2)
draw.text((10, 10), "A", "#f00", font, strokeWidth=2, strokeFill="#0f0")
draw.multiline_text((10, 10), "A\nB", "#f00", font, strokeWidth=2, strokeFill="#0f0")

Uses FreeType's stroking. See https://www.freetype.org/freetype2/docs/reference/ft2-glyph_stroker.html for more information, and part of https://www.freetype.org/freetype2/docs/tutorial/example2.cpp for example code.

Also renames a FT_GlyphSlot variable from 'glyph' to 'glyphSlot'.

[hugovk]

And update the py:methods for the other updated ones.

[hugovk]

Just a naming thing, then we're good to merge. Thanks!