imageviewer: limit progress bar LCD update rate

The progress callback doesn't rate limit LCD updates, which causes
excessive slowdowns (up to 150x!) on some targets. Limiting updates
to 20fps solves the problem and should boost load speed across the
board, particularly for large images and animated GIFs that may run
the progress callback thousands of times.

Change-Id: Ia48924d4dcb24c1111509329581809be494d0165

apps/plugins/imageviewer/imageviewer.c

@@ -127,6 +127,9 @@ static int curfile = -1, direction = DIR_NEXT, entries = 0;
 /* list of the supported image files */
 static char **file_pt;
 
+/* progress update tick */
+static long next_progress_tick;
+
 static const struct image_decoder *imgdec = NULL;
 static enum image_type image_type = IMAGE_UNKNOWN;
 
@@ -428,7 +431,14 @@ static int ask_and_get_audio_buffer(const char *filename)
 /* callback updating a progress meter while image decoding */
 static void cb_progress(int current, int total)
 {
-    rb->yield(); /* be nice to the other threads */
+    /* do not yield or update the progress bar if we did so too recently */
+    long now = *rb->current_tick;
+    if(!TIME_AFTER(now, next_progress_tick))
+        return;
+
+    /* limit to 20fps */
+    next_progress_tick = now + HZ/20;
+
 #ifndef USEGSLIB
     /* in slideshow mode, keep gui interference to a minimum */
     const int size = (!iv_api.running_slideshow ? 8 : 4);
@@ -442,6 +452,8 @@ static void cb_progress(int current, int total)
                             total, 0, current, HORIZONTAL);
         rb->lcd_update_rect(0, LCD_HEIGHT-size, LCD_WIDTH, size);
     }
+
+    rb->yield(); /* be nice to the other threads */
 }
 
 #define VSCROLL (LCD_HEIGHT/8)