Buffer queue: Correct behavior of free buffer.

This corrects the behavior of free buffer after witnessing it in an
unrelated hardware test. I haven't found any games affected by it but in
name of better accuracy we'll correct such behavior.

src/core/hle/service/nvflinger/buffer_queue.cpp

@@ -28,6 +28,7 @@ void BufferQueue::SetPreallocatedBuffer(u32 slot, const IGBPBuffer& igbp_buffer)
     buffer.slot = slot;
     buffer.igbp_buffer = igbp_buffer;
     buffer.status = Buffer::Status::Free;
+    free_buffers.push_back(slot);
 
     queue.emplace_back(buffer);
     buffer_wait_event.writable->Signal();
@@ -35,17 +36,38 @@ void BufferQueue::SetPreallocatedBuffer(u32 slot, const IGBPBuffer& igbp_buffer)
 
 std::optional<std::pair<u32, Service::Nvidia::MultiFence*>> BufferQueue::DequeueBuffer(u32 width,
                                                                                        u32 height) {
-    auto itr = std::find_if(queue.begin(), queue.end(), [&](const Buffer& buffer) {
-        // Only consider free buffers. Buffers become free once again after they've been Acquired
-        // and Released by the compositor, see the NVFlinger::Compose method.
+
+    if (free_buffers.empty()) {
+        return {};
+    }
+
+    auto f_itr = free_buffers.begin();
+    auto itr = queue.end();
+
+    while (f_itr != free_buffers.end()) {
+        auto slot = *f_itr;
+        itr = std::find_if(queue.begin(), queue.end(), [&](const Buffer& buffer) {
+            // Only consider free buffers. Buffers become free once again after they've been
+            // Acquired and Released by the compositor, see the NVFlinger::Compose method.
             if (buffer.status != Buffer::Status::Free) {
                 return false;
             }
 
+            if (buffer.slot != slot) {
+                return false;
+            }
+
             // Make sure that the parameters match.
             return buffer.igbp_buffer.width == width && buffer.igbp_buffer.height == height;
         });
 
+        if (itr != queue.end()) {
+            free_buffers.erase(f_itr);
+            break;
+        }
+        ++f_itr;
+    }
+
     if (itr == queue.end()) {
         return {};
     }
@@ -99,6 +121,7 @@ void BufferQueue::ReleaseBuffer(u32 slot) {
     ASSERT(itr != queue.end());
     ASSERT(itr->status == Buffer::Status::Acquired);
     itr->status = Buffer::Status::Free;
+    free_buffers.push_back(slot);
 
     buffer_wait_event.writable->Signal();
 }

src/core/hle/service/nvflinger/buffer_queue.h

@@ -101,6 +101,7 @@ private:
     u32 id;
     u64 layer_id;
 
+    std::list<u32> free_buffers;
     std::vector<Buffer> queue;
     std::list<u32> queue_sequence;
     Kernel::EventPair buffer_wait_event;