Improve PlayHandle removal in Mixer

[M374LX]

Problem

In Mixer::renderNextBuffer(), when a clear signal is received (by calling Mixer::clear(), which sets m_clearSignal to true), Mixer::clearInternal() gets called and it simply adds every PlayHandle except InstrumentPlayHandles from m_playHandles to m_playHandlesToRemove. However, right after Mixer::clearInternal() is called in Mixer::renderNextBuffer(), a loop iterates through m_playHandlesToRemove and removes the same PlayHandles from m_playHandles. In other words, it unnecessarily stores a list of PlayHandles to be removed only to actually remove then right afterwards. Mixer::clear() seems to be called only within Song::stop().

Other than Mixer::clear(), the only method that adds a play handle to m_playHandlesToRemove is Mixer::removePlayHandle(), which is only called by the FileBrowser class to stop a preset preview PlayHandle. Also, only one preset preview PlayHandle seems to be active at a time.

Solution

1. Add the members PresetPreviewPlayHandle m_presetPreviewPlayHandle and bool m_presetPreviewStopped to Mixer.

2. Create the method Mixer::addPresetPreviewPlayHandle():

bool Mixer::addPresetPreviewPlayHandle( PresetPreviewPlayHandle *handle )
{
	m_presetPreviewStopped = false;
	m_presetPreviewPlayHandle = handle;
	return addPlayHandle( handle );
}

3. Create the method Mixer::stopPresetPreview():

void Mixer::stopPreview()
{
	m_presetPreviewStopped = true;
}

4. Modify Mixer::renderNextBuffer() so that:

if( m_clearSignal )
{
	m_clearSignal = false;
	clearInternal();
}

ConstPlayHandleList::Iterator it_rem = m_playHandlesToRemove.begin();
while( it_rem != m_playHandlesToRemove.end() )
{
	// [...]
}

becomes something like:

if( m_presetPreviewStopped && m_presetPreviewPlayHandle != NULL ) {
	//Actually stop m_presetPreviewPlayHandle [...]
	m_presetPreviewPlayHandle = NULL;
}

if( m_clearSignal )
{
	ConstPlayHandleList::Iterator it = m_playHandles.begin();
	while( it != m_playHandles.end() )
	{
		// [...]
	}

	m_clearSignal = false;
}

5. Update FileBrowser.cpp with the new methods.

[PhysSong]

Mixer::clearInternal was added in #2879 by @jasp00 to fix a deadlock. I think we can remove it once we find a solution without deadlocks.
For preset previewing, I'm not sure why that can be an issue. I also think Mixer is not a place for handling preset previews.

[Rossmaxx]

Mixer has been rewritten recently. Does this issue still occur?