show preview of textures during editing

When textures of the map geometry are changed, five textures around the
current pick from the "last used" stack are shown. The texture assigned
to the "sky" slot is not shown. Instead, a plain blue rectangle is used.

hudtexttl
  Time to show texture preview (in milliseconds).

DOCREF hudtexttl

source/src/renderhud.cpp

@@ -1054,6 +1054,9 @@ void gl_drawhud(int w, int h, int curfps, int nquads, int curvert, bool underwat
         if(!m_botmode) draw_textf("SCROLL to change player", left - (text_width("SCROLL to change player") + FONTH/2), top+80);
     }
 
+    void renderhudtexturepreviews();
+    if(editmode) renderhudtexturepreviews();
+
     /* * /
     glLoadIdentity();
     glOrtho(0, VIRTW*3/2, VIRTH*3/2, 0, -1, 1);