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/terminal_view/src/terminal_element.rs

@@ -662,7 +662,8 @@ impl Element for TerminalElement {
                 let dimensions = {
                     let rem_size = window.rem_size();
                     let font_pixels = text_style.font_size.to_pixels(rem_size);
-                    let line_height = font_pixels * line_height.to_pixels(rem_size);
+                    // TODO: line_height should be an f32 not an AbsoluteLength.
+                    let line_height = font_pixels * line_height.to_pixels(rem_size).0;
                     let font_id = cx.text_system().resolve_font(&text_style.font());
 
                     let cell_width = text_system