Implement wrap-around scrolling (closes #44)

Applied a modified version of raylu's patch. One can enable/disable wrap-around
scrolling with `set scroll_wrap 1/0". It's disabled per default.

config.def.h

@@ -66,6 +66,7 @@ char* uri_command = "firefox %s"; /* uri */
 
 /* additional settings */
 gboolean show_scrollbars = FALSE;
+gboolean scroll_wrap = FALSE;
 int adjust_open          = ADJUST_BESTFIT;
 #define SELECTION_STYLE POPPLER_SELECTION_GLYPH
 #define GOTO_MODE GOTO_LABELS /* GOTO_DEFAULT, GOTO_LABELS, GOTO_OFFSET */
@@ -245,6 +246,7 @@ Setting settings[] = {
   {"recolor_darkcolor",      &(recolor_darkcolor),               's',   FALSE,   TRUE,    "Recoloring (dark color)"},
   {"recolor_lightcolor",     &(recolor_lightcolor),              's',   FALSE,   TRUE,    "Recoloring (light color)"},
   {"scroll_step",            &(scroll_step),                     'f',   FALSE,   FALSE,   "Scroll step"},
+  {"scroll_wrap",            &(scroll_wrap),                     'b',   FALSE,   FALSE,   "Wrap scolling at last page"},
   {"scrollbars",             &(show_scrollbars),                 'b',   FALSE,   TRUE,    "Show scrollbars"},
   {"show_statusbar",         &(Zathura.Global.show_statusbar),   'b',   FALSE,   TRUE,    "Show statusbar"},
   {"show_inputbar",          &(Zathura.Global.show_inputbar),    'b',   FALSE,   TRUE,    "Show inputbar"},

zathura.c

@@ -2092,9 +2092,12 @@ sc_navigate(Argument* argument)
   int new_page = Zathura.PDF.page_number;
 
   if(argument->n == NEXT)
-    new_page = (new_page + number_of_pages + 1) % number_of_pages;
+    new_page = scroll_wrap ? (new_page + 1) : ((new_page + 1) % number_of_pages);
   else if(argument->n == PREVIOUS)
-    new_page = (new_page + number_of_pages - 1) % number_of_pages;
+    new_page = scroll_wrap ? (new_page - 1) : ((new_page + number_of_pages - 1) % number_of_pages);
+
+  if (scroll_wrap && (new_page < 0 || new_page >= number_of_pages))
+    return;
 
   set_page(new_page);
   update_status();
@@ -2172,12 +2175,16 @@ sc_scroll(Argument* argument)
 
   if((argument->n == UP || argument->n == HALF_UP || argument->n == FULL_UP) && value == 0)
   {
+    int old_page = Zathura.PDF.page_number;
     Argument arg;
     arg.n = PREVIOUS;
     sc_navigate(&arg);
-    arg.n = BOTTOM;
-    ss = TRUE;
-    sc_scroll(&arg);
+    if (!scroll_wrap || (Zathura.PDF.page_number < old_page))
+    {
+      arg.n = BOTTOM;
+      ss = TRUE;
+      sc_scroll(&arg);
+    }
     return;
   }
   else if((argument->n == DOWN || argument->n == HALF_DOWN || argument->n == FULL_DOWN) && value == max)