fix: dangling OpenGL context crash and uninitialized audio frame size

- OpenGLRenderer: hold the viewer-owned QOpenGLContext in a QPointer so
  DestroyInternal() safely skips it when the context has already been
  destroyed by Qt's shared-context lifecycle. Fixes a SIGSEGV when the
  full gtest suite ran MainWindow.ConstructsOffscreenWithPanelsAndMenus
  after earlier viewer tests.
- PreviewAudioDevice: add SetParams() deriving bytes_per_frame from the
  audio format (bytes per sample * channel count) instead of staying 0.

app/render/previewaudiodevice.h

@@ -22,6 +22,8 @@
 #ifndef PREVIEWAUDIODEVICE_H
 #define PREVIEWAUDIODEVICE_H
 
+#include <olive/core/render/audioparams.h>
+
 #include "previewautocacher.h"
 
 namespace olive
@@ -42,6 +44,11 @@ public:
 
 	virtual qint64 writeData(const char *data, qint64 length) override;
 
+	// Derives the frame size from the audio format (bytes per sample per
+	// channel * channel count). Until params are set, bytes_per_frame()
+	// reports 0, i.e. "unknown".
+	void SetParams(const core::AudioParams &params);
+
 	int bytes_per_frame() const
 	{
 		return bytes_per_frame_;