rendering: ceil() instead of truncating for tabbed deco_rect width (Thanks szalik)

In tabbed mode, the available width (say 1280) is divided by the amount
of child containers (say 3). Before this commit, we just truncated the
result and would end up with 426 + 426 + 426 = 1278 pixels that we
render to. Now we render a bit too much, but that’ll at least not give
us graphics corruption on any side :).

fixes #791

src/render.c

@@ -345,7 +345,7 @@ void render_con(Con *con, bool render_fullscreen) {
             child->rect.width = rect.width;
             child->rect.height = rect.height;
 
-            child->deco_rect.width = child->rect.width / children;
+            child->deco_rect.width = ceil((float)child->rect.width / children);
             child->deco_rect.x = x - con->rect.x + i * child->deco_rect.width;
             child->deco_rect.y = y - con->rect.y;