allow configuring imlib2's cache size (#171)

by default imlib2 uses a 4mb cache, which is quite small. this allows
users who have more memory to spare to set a bigger cache size and avoid
reloading an already viewed image if it fits into the cache.

Co-authored-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>

image.c

@@ -48,6 +48,7 @@ void img_init(img_t *img, win_t *win)
 	imlib_context_set_display(win->env.dpy);
 	imlib_context_set_visual(win->env.vis);
 	imlib_context_set_colormap(win->env.cmap);
+	imlib_set_cache_size(CACHE_SIZE);
 
 	img->im = NULL;
 	img->win = win;