Engine: when drawing TextWindow clip border pieces, fix over corners

This fixes vertical or horizontal borders to be drawn under rightmost and bottom corners, which may have transparent parts.
Doing this for 3.6.1+ games, because in theory older games could have been adjusted specifically to use or counter this behavior.

Engine/ac/display.cpp

@@ -665,8 +665,10 @@ void draw_button_background(Bitmap *ds, int xx1,int yy1,int xx2,int yy2,GUIMain*
         if (iep->BgColor > 0)
             ds->FillRect(Rect(xx1,yy1,xx2,yy2), draw_color);
 
-        int leftRightWidth = game.SpriteInfos[get_but_pic(iep,4)].Width;
-        int topBottomHeight = game.SpriteInfos[get_but_pic(iep,6)].Height;
+        const int leftRightWidth = game.SpriteInfos[get_but_pic(iep,4)].Width;
+        const int topBottomHeight = game.SpriteInfos[get_but_pic(iep,6)].Height;
+        const bool clip_borders = loaded_game_file_version >= kGameVersion_361;
+        // GUI middle space
         if (iep->BgImage>0) {
             {
                 // offset the background image and clip it so that it is drawn
@@ -692,14 +694,22 @@ void draw_button_background(Bitmap *ds, int xx1,int yy1,int xx2,int yy2,GUIMain*
                 ds->ResetClip();
             }
         }
+        // Vertical borders
+        if (clip_borders)
+            ds->SetClip(Rect(xx1 - leftRightWidth, yy1, xx2 + 1 + leftRightWidth, yy2));
         for (int uu=yy1;uu <= yy2;uu+= game.SpriteInfos[get_but_pic(iep,4)].Height) {
             do_corner(ds, get_but_pic(iep,4),xx1,uu,-1,0);   // left side
             do_corner(ds, get_but_pic(iep,5),xx2+1,uu,0,0);  // right side
         }
+        // Horizontal borders
+        if (clip_borders)
+            ds->SetClip(Rect(xx1, yy1 - topBottomHeight, xx2, yy2 + 1 + topBottomHeight));
         for (int uu=xx1;uu <= xx2;uu+=game.SpriteInfos[get_but_pic(iep,6)].Width) {
             do_corner(ds, get_but_pic(iep,6),uu,yy1,0,-1);  // top side
             do_corner(ds, get_but_pic(iep,7),uu,yy2+1,0,0); // bottom side
         }
+        ds->ResetClip();
+        // Four corners
         do_corner(ds, get_but_pic(iep,0),xx1,yy1,-1,-1);  // top left
         do_corner(ds, get_but_pic(iep,1),xx1,yy2+1,-1,0);  // bottom left
         do_corner(ds, get_but_pic(iep,2),xx2+1,yy1,0,-1);  //  top right