Don't copy any VST3 state preamble

Bitwig prepends some data when passing an `IBStream*` to the plugin, and
when we do copy it iZotope Rx7 plugins cannot load their state (even if
we also copy over the same seek position). Not copying that preamble
fixes the issue, and it seems like it doesn't break anything.

CHANGELOG.md

@@ -105,6 +105,7 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - Fixed another potential freezing issue in **REAPER** that could happen when the
   when the plugin resizes itself while sending a parameter change to the host,
   if REAPER's 'disable saving full plug-in state' is not disabled.
+- Fixed saving and loading _iZotope Rx7_ plugin state in **Bitwig Studio**.
 - Fixed a potential assertion failure when setting VST3 preset data. This would
   depend on the `libstdc++` version used to built yabridge with.
 - _PSPaudioware InifniStrip_ would fail to initialize because the plugin expects

src/common/serialization/vst3/bstream.cpp

@@ -30,21 +30,31 @@ YaBStream::YaBStream(Steinberg::IBStream* stream) {
 
     // Copy any existing contents, used for `IComponent::setState` and similar
     // methods
+    // NOTE: Bitwig Studio seems to prepend some default header on new presets
+    //       We _don't_ want to copy that, since some plugins may try to read
+    //       the entire preset and fail to load. Examples of such plugins are
+    //       the iZotope Rx7 plugins.
     int64 old_position;
     stream->tell(&old_position);
     if (stream->seek(0, Steinberg::IBStream::IStreamSeekMode::kIBSeekEnd) ==
         Steinberg::kResultOk) {
         int64 size = 0;
         stream->tell(&size);
+        size -= old_position;
+
         if (size > 0) {
             int32 num_bytes_read = 0;
             buffer.resize(size);
-            stream->seek(0, Steinberg::IBStream::IStreamSeekMode::kIBSeekSet);
+            stream->seek(old_position,
+                         Steinberg::IBStream::IStreamSeekMode::kIBSeekSet);
             stream->read(buffer.data(), static_cast<int32>(size),
                          &num_bytes_read);
             assert(num_bytes_read == 0 || num_bytes_read == size);
         }
     }
+
+    // If the host did prepend some header, we should leave it in place when
+    // writing
     stream->seek(old_position,
                  Steinberg::IBStream::IStreamSeekMode::kIBSeekSet);