fix banner support for 5.12+ (#11)

* fix banner support for 5.12+

- changed from .x to .obj format
- exported at 0.2 scale -> the base is still a bit larger
  than a node but exporting smaller causes the base to levatate.

* fix pole

- moved up
- changed from .x to .obj format

* fix preview

by resizing the background the scaling works again.
Though lua_api.md promises that lower resolution textures
are upscaled to larger ones [see TEXMOD_UPSCALE]

* tweak formspec layout

nodes.lua

@@ -1,7 +1,7 @@
 -- da wooden banner
 core.register_node("banners:wooden_banner", {
     drawtype = "mesh",
-    mesh = "banner_support.x",
+    mesh = "banner_support.obj",
     tiles = { "banner_support.png" },
     description = "Wooden banner",
     groups = { choppy = 2, dig_immediate = 2 },
@@ -20,7 +20,7 @@ core.register_node("banners:wooden_banner", {
 -- steel banner
 core.register_node("banners:steel_banner", {
     drawtype = "mesh",
-    mesh = "banner_support.x",
+    mesh = "banner_support.obj",
     tiles = { "steel_support.png" },
     description = "Steel banner",
     groups = { cracky = 2 },