Add z_index parameter

DOCS.md

@@ -13,6 +13,7 @@ The parameters it takes are the following:
 * `position`: the panel position (as in any other HUD)
 * `alignment`: same
 * `offset`: same
+* `z_index`: as in the Minetest HUD API
 * `bg`: the picture to put in the background
 * `bg_scale`: its scaling
 * `title`: the default text. Default is empty (`""`)

api.lua

@@ -21,6 +21,7 @@ Panel = {
         alignment = { x = 0, y = 0 },
         offset    = {x = 0, y = 0},
         text = "panel_bg.png",
+        z_index = 0
     },
     title_def = {
         hud_elem_type = "text",
@@ -29,7 +30,8 @@ Panel = {
         offset    = {x = 0, y = 0},
         size      = { x = 1 },
         number    = 0xFFFFFF,
-        text      = ""
+        text      = "",
+        z_index   = 0
     },
 
     sub_img_elems = {},       -- KEY: ID, VALUE: name
@@ -81,6 +83,11 @@ function Panel:new(name, def)
       panel.title_def.offset = def.offset
     end
 
+    if def.z_index then
+      panel.background_def.z_index = def.z_index
+      panel.title_def.z_index = def.z_index
+    end
+
     if def.bg then
       panel.background_def.text = def.bg
     end