Add support for embedding data in generated C++ code

This allows compiling with SIXTYFPS_EMBED_RESOURCES=true and
images/fonts are embedded as inline variables.

Generated data is emitted into the header file as

   inline uint8_t sfps_embedded_resources_123[789] = {
       0x1, 0x2, 0x3,
   };

api/sixtyfps-cpp/include/sixtyfps_image.h

@@ -69,6 +69,9 @@ public:
     /// Returns false if \a a refers to the same image as \a b; true otherwise.
     friend bool operator!=(const Image &a, const Image &b) { return a.data != b.data; }
 
+    /// \private
+    explicit Image(cbindgen_private::types::Image inner) : data(inner) { }
+
 private:
     using Tag = cbindgen_private::types::ImageInner::Tag;
     using Data = cbindgen_private::types::Image;