If left and right sides meet in ImageDraw.rounded_rectangle(), do not draw rectangle to fill gap

[radarhere]

Resolves #8303

rounded_rectangle() draws

1. the corners
2. a vertical rectangle to join the left and right sides
3. a rectangle on the left to join the top left and lower left corners
4. a rectangle on the right to join the top right and lower right corners

The code from the issue is failing in step 2 when there is no gap between the left and the right sides. So this PR skips that step if it isn't needed.