Allow explicit zero width to hide outline

[radarhere]

Helps #4277

ImageDraw operations chord, ellipse, pieslice and rectangle take optional arguments outline with a default of None and width with a default of 0.

At present, specifying an outline and a width of zero stills draws the outline. The following three lines all produce the same result -

draw.rectangle(BBOX2, fill="green", outline="yellow")
draw.rectangle(BBOX2, fill="green", outline="yellow", width=0)
draw.rectangle(BBOX2, fill="green", outline="yellow", width=1)

This PR changes the behaviour so that a zero width produces no outline. For backwards compatibility, it changes the width default to 1, so that these two lines still produce the same result -

draw.rectangle(BBOX2, fill="green", outline="yellow")
draw.rectangle(BBOX2, fill="green", outline="yellow", width=1)

but

draw.rectangle(BBOX2, fill="green", outline="yellow", width=0)

does not.

[hugovk]

Perhaps we should mention the default change in the release notes and docstring?

Or maybe it's not needed because the functionality has been fixed?

[radarhere]

...I'm inclined to suggest no?

Documenting that we have changed the default argument would make you presume that we changed the default behaviour, which we haven't. draw.rectangle(BBOX2, fill="green", outline="yellow") still has the same output before and after this PR. So that might actually introduce confusion.