Add RectShape::stroke_kind to control if stroke is inside/outside/centered

[emilk]

Adds RectShape::stroke_kind so you can select if the stroke goes inside, outside, or is centered on the rectangle.

Also adds RectShape::round_to_pixels so you can override TessellationOptions::round_rects_to_pixels.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5647-emilkcustomize-rect-shape
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.