Add RectShape::blur_width to implement shadows (#4267)

This is mostly a refactor, but has some performance benefits:
* We (re)use the same tessellator as for everything else, leading to
less allocations
* We cull shapes before rendering them

Adding `RectShape::blur_width` means it can also be used for other
effects, such as glow.

crates/epaint/src/shape_transform.rs

@@ -37,6 +37,7 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             rounding: _,
             fill,
             stroke,
+            blur_width: _,
             fill_texture_id: _,
             uv: _,
         })