perf: disable CWin32ReadOnlyFile to prevent duplicate stat calls

* when a file doesn't exist in a search path, then CWin32ReadOnlyFile can't
  open that file. and then we try yet another open in CStdioFile
* technically, we could refactor the open code to be able to communicate
  an error code, but I didn't see much point to using Win32 specific files
* I think there are some cases where not using CWin32ReadOnlyFile can break
  some demo playback on Windows, so I might either fix that or do the above

Author: mastercoms

src/filesystem/filesystem_stdio.cpp

@@ -240,8 +240,8 @@ ConVar filesystem_unbuffered_io( "filesystem_unbuffered_io", "1", 0, "" );
 #define UseUnbufferedIO() true
 #endif
 
-ConVar filesystem_native( "filesystem_native", "1", 0, "Use native FS or STDIO" );
-ConVar filesystem_max_stdio_read( "filesystem_max_stdio_read", IsX360() ? "64" : "16", 0, "" );
+ConVar filesystem_native( "filesystem_native", "0", 0, "Use native FS or STDIO" );
+ConVar filesystem_max_stdio_read( "filesystem_max_stdio_read", "64", 0, "" );
 ConVar filesystem_report_buffered_io( "filesystem_report_buffered_io", "0" );
 
 //-----------------------------------------------------------------------------
@@ -417,7 +417,7 @@ FILE *CFileSystem_Stdio::FS_fopen( const char *filenameT, const char *options, u
 
 	CBaseFileSystem::FixUpPath ( filenameT, filename, sizeof( filename ) );
 
-#ifdef _WIN32
+#if defined(_WIN32) && 0
 	if ( CWin32ReadOnlyFile::CanOpen( filename, options ) )
 	{
 		pFile = CWin32ReadOnlyFile::FS_fopen( filename, options, size );