Fix D3D z-buffer problem by having separate GetOrthoMatrix functions for OGL and D3D.

I don't understand why the two APIs create an ortho matrix so differently, but I'm tried of messing with it for now.

stepmania/src/RageDisplay_OGL.cpp

@@ -999,3 +999,13 @@ void RageDisplay::SetAlphaTest( bool b )
 	else
 		glDisable( GL_ALPHA_TEST );
 }
+
+RageMatrix RageDisplay::GetOrthoMatrix( float l, float r, float b, float t, float zn, float zf )
+{
+	RageMatrix m(
+		2/(r-l),      0,            0,           0,
+		0,            2/(t-b),      0,           0,
+		0,            0,            -2/(zf-zn),   0,
+		-(r+l)/(r-l), -(t+b)/(t-b), -(zf+zn)/(zf-zn),  1 );
+	return m;
+}