Make pan fraction configurable

config.def.h

@@ -40,6 +40,9 @@ enum { SLIDESHOW_DELAY = 5 };
 static const double GAMMA_MAX   = 10.0;
 static const int    GAMMA_RANGE = 32;
 
+/* command i_scroll pans image 1/PAN_FRACTION of screen width/height */
+static const int PAN_FRACTION = 5;
+
 /* if false, pixelate images at zoom level != 100%,
  * toggled with 'a' key binding
  */

image.c

@@ -590,7 +590,7 @@ bool img_move(img_t *img, float dx, float dy)
 bool img_pan(img_t *img, direction_t dir, int d)
 {
 	/* d < 0: screen-wise
-	 * d = 0: 1/5 of screen
+	 * d = 0: 1/PAN_FRACTION of screen
 	 * d > 0: num of pixels
 	 */
 	float x, y;
@@ -598,8 +598,8 @@ bool img_pan(img_t *img, direction_t dir, int d)
 	if (d > 0) {
 		x = y = MAX(1, (float) d * img->zoom);
 	} else {
-		x = img->win->w / (d < 0 ? 1 : 5);
-		y = img->win->h / (d < 0 ? 1 : 5);
+		x = img->win->w / (d < 0 ? 1 : PAN_FRACTION);
+		y = img->win->h / (d < 0 ? 1 : PAN_FRACTION);
 	}
 
 	switch (dir) {