apply Tatsh's patch from http://ssc.ajworld.net/sm-ssc/bugtracker/view.php?id=218

Docs/credits.txt

@@ -109,6 +109,10 @@ juanelote
 NitroX72
  * Pump/frame noteskin
 
+Tatsh
+ * A patch to fix building with system ffmpeg
+ (http://ssc.ajworld.net/sm-ssc/bugtracker/view.php?id=218)
+
 ==the beta testers==
 [SSC Beta Testing Team]
 KeithD

src/arch/MovieTexture/MovieTexture_FFMpeg.cpp

@@ -685,7 +685,7 @@ int URLRageFile_read( avcodec::URLContext *h, unsigned char *buf, int size )
 	return f->Read( buf, size );
 }
 
-int URLRageFile_write( avcodec::URLContext *h, unsigned char *buf, int size )
+int URLRageFile_write( avcodec::URLContext *h, const unsigned char *buf, int size )
 {
 	RageFileBasic *f = (RageFileBasic *) h->priv_data;
 	return f->Write( buf, size );