Add a method for pixel size in sophus::ImageView

Without this method it's difficult to calculate the actual size in bytes
of subviews because they normally inherit the layout from the parent
image, which has a pitch_bytes indicating the full width times pixel
size.

This simplifies that to expose the pixel size directly.

cpp/sophus/image/image_view.h

@@ -94,6 +94,9 @@ struct ImageView {
   [[nodiscard]] auto sizeBytes() const -> size_t {
     return layout().sizeBytes();
   }
+  [[nodiscard]] auto pixelSize() const -> size_t {
+    return sizeof(Pixel);
+  }
 
   /// Returns true if u is in [0, width).
   [[nodiscard]] auto colInBounds(int u) const -> bool {