This is a breaking change, requiring users to think about wether the
stroke is inside/centered/outside the rect.
When in doubt, add `egui::StrokeKind::Inside` to the function call.
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`.
Emil Ernerfeldt