Added ImageDraw rounded_rectangle method

[radarhere]

Resolves #4765 by adding rounded_rectangle().

Like rectangle(),

def rectangle(self, xy, fill=None, outline=None, width=1):

except with a radius argument

def rounded_rectangle(self, xy, radius=0, fill=None, outline=None, width=1):

In this implementation, I limit the radius of the corners so that it is not greater than half of the width or the height - so a rounded rectangle might be a circle, but not another ellipse.

[radarhere]

I've added a commit to only draw each pixel once, so that translucent colours can be used without creating an inconsistent opacity - see #5183 and #4333 for similar changes.

[glenn-jocher]

+1 for this PR, I like the idea. If it's not too slow I think we will use it in our YOLOv5 repo, as then we can achieve uniformity with our iOS app rectangles with rounded corners.

[hugovk]

Please could you add this to the release notes?

[hugovk]

Looks like this directive is applied to the width param:

Shall we move it up, similar to ImageDraw.textlength, to apply to the whole method?

[radarhere]

Thanks, done.

[hugovk]

Did this get missed in the rebase?

[radarhere]

I don't believe so. I suspect if you refresh https://pillow--5208.org.readthedocs.build/en/5208/reference/ImageDraw.html again, it will work.

[hugovk]

Ah yes, refresh sorted it, thanks!

[radarhere]

Ok, I've added release notes.