Remove Pixels: Mul<Pixels, Output = Pixels> impl, add ScaledPixels ops (#27451)

It doesn't make sense to have `Pixels: Mul<Pixels, Output = Pixels>` as
the output should be `Pixels^2` (area), so these impls are removed. All
code where these impls were used are improved by instead multiplying by
`f32` or `usize`.

Also adds math op impls that are present for `Pixels` but absent for
`ScaledPixels`. Adds missing `Mul<Pixels> for usize` to both.

Release Notes:

- N/A

crates/ui/src/components/indent_guides.rs

@@ -172,10 +172,10 @@ mod uniform_list {
                     .map(|layout| RenderedIndentGuide {
                         bounds: Bounds::new(
                             point(
-                                px(layout.offset.x as f32) * self.indent_size,
-                                px(layout.offset.y as f32) * item_height,
+                                layout.offset.x * self.indent_size,
+                                layout.offset.y * item_height,
                             ),
-                            size(px(1.), px(layout.length as f32) * item_height),
+                            size(px(1.), layout.length * item_height),
                         ),
                         layout,
                         is_active: false,