audren: Make use of nodiscard, rework downmixing, release all buffers

Preliminary work for upmixing & general cleanup. Fixes basic issues in games such as Shovel Knight and slightly improves the LEGO games. Upmixing stitll needs to be implemented.

Audio levels in a few games will be fixed as we now use the downmix coefficients when possible instead of supplying our own

src/audio_core/stream.cpp

@@ -136,4 +136,13 @@ std::vector<Buffer::Tag> Stream::GetTagsAndReleaseBuffers(std::size_t max_count)
     return tags;
 }
 
+std::vector<Buffer::Tag> Stream::GetTagsAndReleaseBuffers() {
+    std::vector<Buffer::Tag> tags;
+    while (!released_buffers.empty()) {
+        tags.push_back(released_buffers.front()->GetTag());
+        released_buffers.pop();
+    }
+    return tags;
+}
+
 } // namespace AudioCore