Add linear interpolation of rendered entities

I've added a function Graphics::lerp() which simply interpolates between
two values given a certain alpha value. It's just like drawing a
straight line between two points.

Also, Graphics now has an `alpha` attribute, and it is set on every
deltatime update to be used in linear interpolation.

desktop_version/src/main.cpp

@@ -543,6 +543,7 @@ int main(int argc, char *argv[])
             game.gameclock();
         }
         const float alpha = static_cast<float>(accumulator) / timesteplimit;
+        graphics.alpha = alpha;
 
         if (game.infocus)
         {