headless_renderer example overallocates Buffer. (#24005)

# Objective

headless_renderer example over allocates the Buffer

`RenderDevice::align_copy_bytes_per_row` takes bytes per row, the
example is passing pixels - then multiplying the padded result by bytes
per pixel.
So it allocates 8192 bytes per row instead of 7680.

## Solution

Multiply pixels per row by bytes per pixel and pass that to
`align_copy_bytes_per_row`

## Testing

- Did you test these changes? If so, how?

Ran the example with various pixel widths.

The over allocation doesn't affect the rendered output because the other
call to `align_copy_bytes_per_row` in `update` is correct, so we
properly extract the image from the padded buffer.

Author: rectalogic

examples/app/headless_renderer.rs

@@ -289,9 +289,7 @@ impl ImageCopier {
         size: Extent3d,
         render_device: &RenderDevice,
     ) -> ImageCopier {
-        let padded_bytes_per_row =
-            RenderDevice::align_copy_bytes_per_row((size.width) as usize) * 4;
-
+        let padded_bytes_per_row = RenderDevice::align_copy_bytes_per_row(size.width as usize * 4);
         let cpu_buffer = render_device.create_buffer(&BufferDescriptor {
             label: None,
             size: padded_bytes_per_row as u64 * size.height as u64,