Interpolate activity zone prompt fading in and out

To make it real smooth, just in case it was noticeable that it only
updated at 1000/34 FPS before (well, except in slowmode, it's really
noticeable THERE).

Also this removes the re-typing out of (game.act_fade/10.0f) for every
single R, G, and B in gamerender().

desktop_version/src/Game.cpp

@@ -150,6 +150,7 @@ void Game::init(void)
     completestop = false;
     activeactivity = -1;
     act_fade = 0;
+    prev_act_fade = 0;
     backgroundtext = false;
     startscript = false;
     newscript = "";

desktop_version/src/Game.h

@@ -327,6 +327,7 @@ public:
     bool backgroundtext;
 
     int activeactivity, act_fade;
+    int prev_act_fade;
 
     bool press_left, press_right, press_action, press_map;
 

desktop_version/src/Logic.cpp

@@ -1516,6 +1516,7 @@ void gamelogic()
     }
 #endif
 
+    game.prev_act_fade = game.act_fade;
     if (game.activeactivity > -1)
     {
         if (game.act_fade < 5)

desktop_version/src/Render.cpp

@@ -1629,21 +1629,22 @@ void gamerender()
         }
     }
 
+    float act_alpha = graphics.lerp(game.prev_act_fade, game.act_fade) / 10.0f;
     if (game.activeactivity > -1)
     {
         game.activity_lastprompt = obj.blocks[game.activeactivity].prompt;
         game.activity_r = obj.blocks[game.activeactivity].r;
         game.activity_g = obj.blocks[game.activeactivity].g;
         game.activity_b = obj.blocks[game.activeactivity].b;
-        graphics.drawtextbox(16, 4, 36, 3, game.activity_r*(game.act_fade/10.0f), game.activity_g*(game.act_fade/10.0f), game.activity_b*(game.act_fade/10.0f));
-        graphics.Print(5, 12, game.activity_lastprompt, game.activity_r*(game.act_fade/10.0f), game.activity_g*(game.act_fade/10.0f), game.activity_b*(game.act_fade/10.0f), true);
+        graphics.drawtextbox(16, 4, 36, 3, game.activity_r*act_alpha, game.activity_g*act_alpha, game.activity_b*act_alpha);
+        graphics.Print(5, 12, game.activity_lastprompt, game.activity_r*act_alpha, game.activity_g*act_alpha, game.activity_b*act_alpha, true);
     }
     else
     {
         if(game.act_fade>5)
         {
-            graphics.drawtextbox(16, 4, 36, 3, game.activity_r*(game.act_fade/10.0f), game.activity_g*(game.act_fade/10.0f), game.activity_b*(game.act_fade/10.0f));
-            graphics.Print(5, 12, game.activity_lastprompt, game.activity_r*(game.act_fade/10.0f), game.activity_g*(game.act_fade/10.0f), game.activity_b*(game.act_fade/10.0f), true);
+            graphics.drawtextbox(16, 4, 36, 3, game.activity_r*act_alpha, game.activity_g*act_alpha, game.activity_b*act_alpha);
+            graphics.Print(5, 12, game.activity_lastprompt, game.activity_r*act_alpha, game.activity_g*act_alpha, game.activity_b*act_alpha, true);
         }
     }