Make cmd_resize_tiling_direction work with pixels

Introduces resize_neighboring_cons in resize.c which is also used by
resize_graphical_handler.

Co-authored-by: Andrew Laucius <andrewla@gmail.com>
Authored original code and tests in #3240. I rewrote most of the
resizing code and fixed the failing tests.

parser-specs/commands.spec

@@ -243,7 +243,7 @@ state RESIZE_TILING:
   'or'
       -> RESIZE_TILING_OR
   end
-      -> call cmd_resize($way, $direction, &resize_px, 10)
+      -> call cmd_resize($way, $direction, &resize_px, 0)
 
 state RESIZE_TILING_OR:
   resize_ppt = number