Add dynamic exposure correction (#12959)

* Add uniform for frame delta time
* Adjust exposure in logarithmic (EV) space
* Add network support and LUA API
* Add testing mod

src/lighting.cpp

@@ -0,0 +1,29 @@
+/*
+Minetest
+Copyright (C) 2021 x2048, Dmitry Kostenko <codeforsmile@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#include "lighting.h"
+
+AutoExposure::AutoExposure()
+        : luminance_min(-3.f),
+        luminance_max(-3.f),
+        exposure_correction(0.0f),
+        speed_dark_bright(1000.f),
+        speed_bright_dark(1000.f),
+        center_weight_power(1.f)
+{}