Rework how progress bar shown with stamps

Was drawing progress bar while DRAWING stamps into the UI
(in https://sourceforge.net/p/tuxpaint/tuxpaint/ci/ad4bfde57c170082d93979a7073f7358872eca21/),
even if they were already loaded and thumbnailed.

Now only doing it when loading a new stamp.  Should hopefully
help with performance.  (h/t Pere for reporting slowness on Android)

docs/en/README.txt

@@ -6,7 +6,7 @@
           Copyright 2002-2020 by various contributors; see AUTHORS.txt
                             http://www.tuxpaint.org/
 
-                         June 14, 2002 - March 28, 2020
+                         June 14, 2002 - April 1, 2020
 
      ----------------------------------------------------------------------
 

docs/en/html/README.html

@@ -22,7 +22,7 @@ version
 
 <p>June 14, 2002 -
 
-					March 28, 2020</p>
+					April 1, 2020</p>
 
 </center>
 

src/tuxpaint.c

@@ -22,7 +22,7 @@
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   (See COPYING.txt)
 
-  June 14, 2002 - March 15, 2020
+  June 14, 2002 - April 1, 2020
 */
 
 
@@ -7236,6 +7236,8 @@ static void get_stamp_thumb(stamp_type * sd)
 
   /* nope, see if there's a pre-rendered one we can use */
 
+  show_progress_bar(screen);
+
   need_mirror = sd->mirrored;
   need_flip = sd->flipped;
   bigimg = NULL;
@@ -8850,7 +8852,6 @@ static void draw_stamps(void)
   for (stamp = stamp_scroll[stamp_group]; stamp < stamp_scroll[stamp_group] + max; stamp++)
     {
       i = stamp - stamp_scroll[stamp_group];
-      show_progress_bar(screen);
 
       dest.x = ((i % 2) * 48) + (WINDOW_WIDTH - 96);
       dest.y = ((i / 2) * 48) + 40 + off_y;