use same zbias in D3D as OpenGL

stepmania/src/Model.cpp

@@ -488,7 +488,6 @@ void Model::DrawPrimitives()
 					DISPLAY->SetSphereEnironmentMapping( mat.alpha.bSphereMapped );
 					// UGLY:  This overrides the Actor's BlendMode
 					DISPLAY->SetTextureModeAdd();
-					//DISPLAY->SetTextureModeModulate();
 					DISPLAY->SetTextureFiltering( true );
 				}
 			}

stepmania/src/RageDisplay_D3D.cpp

@@ -1030,7 +1030,7 @@ void RageDisplay_D3D::SetBlendMode( BlendMode mode )
 
 		/* This is almost exclusively used to draw masks to the Z-buffer.  Make sure
 		 * masks always win the depth test when drawn at the same position. */
-		g_pd3dDevice->SetRenderState( D3DRS_ZBIAS, 1 );
+		g_pd3dDevice->SetRenderState( D3DRS_ZBIAS, 30 );	// this bias is bigger than it needs to be to handle the coplanar case
 
 		break;
 	default:

stepmania/src/RageDisplay_OGL.cpp

@@ -1475,7 +1475,7 @@ void RageDisplay_OGL::SetBlendMode( BlendMode mode )
 
 		/* This is almost exclusively used to draw masks to the Z-buffer.  Make sure
 		 * masks always win the depth test when drawn at the same position. */
-		glDepthRange( 0.0, 0.95 );
+		glDepthRange( 0.0, 0.95 );	// this bias is bigger than it needs to be to handle the coplanar case
 		break;
 	default:
 		ASSERT(0);