Fix overlapping buffers.

For now we just ignore the read position of crossing buffer limits.
This is temporary (until write thread reaches the read position), and
simplifes the code logic.

game/ffmpeg.cc

@@ -89,12 +89,10 @@ void AudioBuffer::operator()(const std::int16_t *data, size_t count, int64_t sam
 }
 
 bool AudioBuffer::prepare(std::int64_t pos) {
-	if (pos < 0) pos = 0;
-	std::unique_lock<std::mutex> l(m_mutex, std::try_to_lock);
-	if (!l.owns_lock()) return false;  // Didn't get lock, give up for now
-	if (eof(pos)) return true;
-	m_read_pos = pos;
-	m_cond.notify_all();
+	// perform fake read to trigger any potential seek
+	if (!read(nullptr, 0, pos, 1)) return true;
+
+	std::unique_lock<std::mutex> l(m_mutex);
 	// Has enough been prebuffered already and is the requested position still within buffer
 	auto ring_size = static_cast<std::int64_t>(m_data.size());
 	return m_write_pos > m_read_pos + ring_size / 16 && m_write_pos <= m_read_pos + ring_size;
@@ -120,10 +118,12 @@ bool AudioBuffer::read(float* begin, size_t samples, std::int64_t pos, float vol
 	}
 
 	std::unique_lock<std::mutex> l(m_mutex);
-	if (eof(pos) || m_quit)
+	if (eof(pos + samples) || m_quit)
 		return false;
 
-	if (pos >= m_read_pos + static_cast<std::int64_t>(m_data.size()) || pos < m_read_pos) {
+	// one cannot read more data than the size of buffer
+	samples = std::min(samples, m_data.size());
+	if (pos >= m_read_pos + static_cast<std::int64_t>(m_data.size() - samples) || pos < m_read_pos) {
 		// in case request position is not in the current possible range, we trigger a seek
 		// Note: m_write_pos is not checked on purpose: if pos is after
 		// m_write_pos, zeros present in buffer will be returned