add SupportsThreadedRendering flag (not in OGL yet)

stepmania/src/RageDisplay.h

@@ -141,6 +141,7 @@ public:
 	virtual void SetBlendMode( BlendMode mode ) = 0;
 
 	virtual bool SupportsTextureFormat( PixelFormat pixfmt, bool realtime=false ) = 0;
+	virtual bool SupportsThreadedRendering() { return false; }
 
 	/* return 0 if failed or internal texture resource handle 
 	 * (unsigned in OpenGL, texture pointer in D3D) */

stepmania/src/RageDisplay_D3D.cpp

@@ -682,6 +682,11 @@ bool RageDisplay_D3D::SupportsTextureFormat( PixelFormat pixfmt, bool realtime )
     return SUCCEEDED( hr );
 }
 
+bool RageDisplay_D3D::SupportsThreadedRendering()
+{
+	return true;
+}
+
 RageSurface* RageDisplay_D3D::CreateScreenshot()
 {
 #if defined(XBOX)

stepmania/src/RageDisplay_D3D.h

@@ -18,6 +18,7 @@ public:
 	VideoModeParams GetVideoModeParams() const;
 	void SetBlendMode( BlendMode mode );
 	bool SupportsTextureFormat( PixelFormat pixfmt, bool realtime=false );
+	bool SupportsThreadedRendering();
 	unsigned CreateTexture( 
 		PixelFormat pixfmt, 
 		RageSurface* img,