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

init.lua

@@ -100,8 +100,8 @@ function banners.creation_form_func(state)
     state.banner:read_item(state.player)
     state.current_color = state.banner.color
     state:size(20, 10)
-    state:image(3, 0.4, 4, 2, "banner_preview", nil)
-    state:image(2.4, 0.8, 0.7, 0.7, "color_indicator", state.current_color)
+    state:image(3.4, 0.4, 4, 2, "banner_preview", nil)
+    state:image(2.55, 0.95, 0.7, 0.7, "color_indicator", state.current_color)
     state:update_preview_inv()
     -- color indicator
     -- undo button
@@ -346,7 +346,7 @@ core.register_entity("banners:banner_ent", {
         collisionbox = { 0, 0, 0, 0, 0, 0 },
         visual = "mesh",
         textures = { "banner_uv_text" },
-        mesh = "banner_pole.x",
+        mesh = "banner_pole.obj",
     },
     on_activate = banners.banner_on_activate,
 })