3D viewer: do not build the board 3D data during 3D frame creation, but after.

Building the 3D data is time consuming, so creating the data after the 3D
frame is shown is better, and the build activity is visible, especially on Linux.

pcbnew/pcb_base_frame.cpp

@@ -452,6 +452,11 @@ EDA_3D_VIEWER_FRAME* PCB_BASE_FRAME::CreateAndShow3D_Frame()
     if( wxWindow::FindFocus() != draw3DFrame )
         draw3DFrame->SetFocus();
 
+    // Allocate a slice of time to display the 3D frame
+    wxSafeYield();
+    // And show the current board
+    Update3DView( true, true );
+
     return draw3DFrame;
 }