Optional: add value_or() helper

Signed-off-by: Roman Lebedev <[email protected]>

Author: LebedevRI

src/librawspeed/adt/Optional.h

@@ -87,6 +87,10 @@ template <typename T> class Optional final {
 
   explicit operator bool() const { return has_value(); }
 
+  template <typename U> T value_or(U&& fallback) const& {
+    return impl.value_or(std::forward<U>(fallback));
+  }
+
   void reset() { impl.reset(); }
 };