ENH: refine the thumbnail rendering

1. add specific shader for thumbnail
2. do not render the negative-z part
3. fix the sinking related issue under cli

Change-Id: Ia59083437544c1c0bd924c811274ccbb137e9eb7

src/slic3r/GUI/GLShadersManager.cpp

@@ -35,6 +35,8 @@ std::pair<bool, std::string> GLShadersManager::init()
 
     // used to render bed axes and model, selection hints, gcode sequential view marker model, preview shells, options in gcode preview
    valid &= append_shader("gouraud_light", { "gouraud_light.vs", "gouraud_light.fs" });
+    //used to render thumbnail
+   valid &= append_shader("thumbnail", { "thumbnail.vs", "thumbnail.fs" });
    // used to render first layer for calibration
    valid &= append_shader("cali", { "cali.vs", "cali.fs"});
     // used to render printbed