libcamera: utils: Add to_underlying() helper function

C++23 has a std::to_underlying() helper function that converts an
enumeration value to its underlying type. Add a compatible
implementation to the libcamera::utils namespace.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>

include/libcamera/base/utils.h

@@ -369,6 +369,12 @@ decltype(auto) abs_diff(const T &a, const T &b)
 
 double strtod(const char *__restrict nptr, char **__restrict endptr);
 
+template<class Enum>
+constexpr std::underlying_type_t<Enum> to_underlying(Enum e) noexcept
+{
+	return static_cast<std::underlying_type_t<Enum>>(e);
+}
+
 } /* namespace utils */
 
 #ifndef __DOXYGEN__