Remove textrect attribute from textboxclass

Instead of duplicating the same variables over and over again,
Graphics::drawgui() can just make its own SDL_Rect. It's not that hard.

As far as I can tell, textrect was always being properly kept up to date
by the time Graphics::drawgui() got around to rendering
(textboxclass::resize() keeps being called a LOT), so this shouldn't be
a noticeable change from the user perspective.

desktop_version/src/Textbox.h

@@ -35,7 +35,6 @@ public:
     int xp, yp, lw, w, h;
     int r,g,b;
     int tr,tg,tb;
-    SDL_Rect textrect;
     int timer;
 
     float tl;