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.cpp

@@ -42,6 +42,11 @@ bool PreviewAudioDevice::isSequential() const
 	return true;
 }
 
+void PreviewAudioDevice::SetParams(const core::AudioParams &params)
+{
+	set_bytes_per_frame(params.samples_to_bytes(1));
+}
+
 qint64 PreviewAudioDevice::readData(char *data, qint64 maxSize)
 {
 	QMutexLocker locker(&lock_);