Implement IStreamAttributes when reading from host

This allows presets to contain meta data about file names and the type
of preset.

Even though the docs don't mention that this is also relevant for
`getState()`, we should also implement it there so plugins can write
their own meta data.

src/common/serialization/vst3/bstream.h

@@ -16,8 +16,12 @@
 
 #pragma once
 
-#include <pluginterfaces/base/ibstream.h>
+#include <optional>
 
+#include <pluginterfaces/base/ibstream.h>
+#include <pluginterfaces/vst/ivstattributes.h>
+
+#include "attribute-list.h"
 #include "base.h"
 
 #pragma GCC diagnostic push
@@ -27,9 +31,13 @@
  * Serialize an `IBStream` into an `std::vector<uint8_t>`, and allow the
  * receiving side to use it as an `IBStream` again. `ISizeableStream` is defined
  * but then for whatever reason never used, but we'll implement it anyways.
+ *
+ * If we're copying data from an existing `IBstream` and that stream supports
+ * VST 3.6.0 preset meta data, then we'll copy that meta data as well.
  */
 class YaBStream : public Steinberg::IBStream,
-                  public Steinberg::ISizeableStream {
+                  public Steinberg::ISizeableStream,
+                  public Steinberg::Vst::IStreamAttributes {
    public:
     YaBStream();
 
@@ -71,15 +79,43 @@ class YaBStream : public Steinberg::IBStream,
     tresult PLUGIN_API getStreamSize(int64& size) override;
     tresult PLUGIN_API setStreamSize(int64 size) override;
 
+    // From `IStreamAttributes`
+    tresult PLUGIN_API getFileName(Steinberg::Vst::String128 name) override;
+    Steinberg::Vst::IAttributeList* PLUGIN_API getAttributes() override;
+
     template <typename S>
     void serialize(S& s) {
         s.container1b(buffer, max_vector_stream_size);
         // The seek position should always be initialized at 0
+
+        s.value1b(supports_stream_attributes);
+        s.ext(file_name, bitsery::ext::StdOptional{},
+              [](S& s, std::u16string& name) {
+                  s.text2b(name, std::extent_v<Steinberg::Vst::String128>);
+              });
+        s.ext(attributes, bitsery::ext::StdOptional{});
     }
 
+    /**
+     * Whether this stream supports `IStreamAttributes`. This will be true if we
+     * copied a stream provided by the host that also supported meta data.
+     */
+    bool supports_stream_attributes = false;
+
+    /**
+     * The stream's name, if this stream supports stream attributes.
+     */
+    std::optional<std::u16string> file_name;
+
    private:
     std::vector<uint8_t> buffer;
     size_t seek_position = 0;
+
+    /**
+     * The stream's meta data if we've copied from a stream that supports meta
+     * data.
+     */
+    std::optional<YaAttributeList> attributes;
 };
 
 #pragma GCC diagnostic pop