Discard Gas mode renders (#3486)

* Discard gas_mode renders

This discards the gas_mode / fog effect from games that use it and allows the games to display without it.  Note that gas mode is still unimplemented and will LOG<CRITICAL>.
This bypasses #3287. (Doesn't fix it)

* fix clang

src/video_core/renderer_opengl/gl_shader_gen.cpp

@@ -1175,7 +1175,8 @@ vec4 secondary_fragment_color = vec4(0.0);
         Core::Telemetry().AddField(Telemetry::FieldType::Session, "VideoCore_Pica_UseGasMode",
                                    true);
         LOG_CRITICAL(Render_OpenGL, "Unimplemented gas mode");
-        UNIMPLEMENTED();
+        out += "discard; }";
+        return out;
     }
 
     out += "gl_FragDepth = depth;\n";