fix: opaque-pointer declarations need qmetatype include and single ownership

The scripted Q_DECLARE_OPAQUE_POINTER sweep missed the QtCore/qmetatype.h
include (cascade of stdlib errors) and declared a few types in two headers
(redefinition of IsPointerDeclaredOpaque). Include qmetatype.h everywhere
and declare each handle in exactly one header.

engine/include/oakengine/encoding.h

@@ -500,6 +500,7 @@ oakengine_encoding_start_audio_recording(const OakEngineEncodingParams *params,
 
 #ifdef __cplusplus
 }
+#include <QtCore/qmetatype.h>
 Q_DECLARE_OPAQUE_POINTER(OakEngineEncodingParams *)
 #endif