MediaExtractor and MediaWriter fix constness

* Fixes an ugly hack, where the codec was initialized in the
InitCheck and prevented the method to be const too.

headers/os/codec/MediaExtractor.h

@@ -28,7 +28,7 @@ public:
 								//BMediaExtractor(BMediaStreamer* streamer);
 								~BMediaExtractor();
 
-			status_t			InitCheck();
+			status_t			InitCheck() const;
 
 			BDataIO*			Source() const;
 
@@ -39,7 +39,7 @@ public:
 			status_t			GetStreamMetaData(int32 stream,
 									BMetaData* data) const;
 
-			int32				CountStreams();
+			int32				CountStreams() const;
 
 			const media_format*	EncodedFormat(int32 stream);
 			int64				CountFrames(int32 stream) const;

headers/os/codec/MediaWriter.h

@@ -22,7 +22,7 @@ public:
 									const media_file_format& fileFormat);
 								~BMediaWriter();
 
-			status_t			InitCheck();
+			status_t			InitCheck() const;
 
 			BDataIO*			Target() const;
 
@@ -54,8 +54,9 @@ private:
 				void*			cookie;
 			};
 
+			status_t 			fInitCheck;
 			BDataIO*			fTarget;
-			BWriter*				fWriter;
+			BWriter*			fWriter;
 			List<StreamInfo>	fStreamInfos;
 			media_file_format	fFileFormat;