Interpolate text box alpha

This makes text boxes fade in and out pretty smoothly.

This requires that the textboxclass::setcol() be in Graphics::drawgui(),
so now it's moved there.

Text box fading is only really noticeable if you're playing in slowmode.

desktop_version/src/Graphics.cpp

@@ -609,6 +609,10 @@ void Graphics::drawgui()
     //Draw all the textboxes to the screen
     for (size_t i = 0; i<textbox.size(); i++)
     {
+        //This routine also updates textbox colors
+        float tl_lerp = lerp(textbox[i].prev_tl, textbox[i].tl);
+        textbox[i].setcol(textbox[i].tr * tl_lerp, textbox[i].tg * tl_lerp, textbox[i].tb * tl_lerp);
+
         if (textbox[i].r == 0 && textbox[i].g == 0 && textbox[i].b == 0)
         {
             if(flipmode)

desktop_version/src/Textbox.cpp

@@ -9,6 +9,7 @@ textboxclass::textboxclass()
     h = 0;
     lw = 0;
     tl = 0;
+    prev_tl = 0;
     tm = 0;
     timer = 0;
 }
@@ -56,6 +57,7 @@ void textboxclass::setcol(int rr, int gg, int bb)
 
 void textboxclass::update()
 {
+    prev_tl = tl;
     if (tm == 0)
     {
         tl += .1f;
@@ -64,7 +66,6 @@ void textboxclass::update()
             tl = 1;
             tm = 1;
         }
-        setcol(int(tr * tl), int(tg * tl), int(tb * tl));
     }
     else if (tm == 2)
     {
@@ -74,7 +75,6 @@ void textboxclass::update()
             tl = 0.5;
             //this textbox will be removed by updatetextboxes() later
         }
-        setcol(int(tr * tl), int(tg * tl), int(tb * tl));
     }
     if (timer > 0)
     {

desktop_version/src/Textbox.h

@@ -40,6 +40,7 @@ public:
     int timer;
 
     float tl;
+    float prev_tl;
     int tm;
 
     int max;