SL-13888 Restore back face culling being accidently removed in SL-12781

Due to conflicts picked up ptolemy's fix manually

indra/newview/llmodelpreview.cpp

@@ -2732,6 +2732,7 @@ BOOL LLModelPreview::render()
 
     LLGLSUIDefault def; // GL_BLEND, GL_ALPHA_TEST, GL_CULL_FACE, depth test
     LLGLDisable no_blend(GL_BLEND);
+    LLGLEnable cull(GL_CULL_FACE);
     LLGLDepthTest depth(GL_FALSE); // SL-12781 disable z-buffer to render background color
     LLGLDisable fog(GL_FOG);