add entity 'on_render' event

res/modules/internal/stdcomp.lua

@@ -68,7 +68,7 @@ return {
         end
     end,
     update = function()
-        for id,entity in pairs(entities) do
+        for _,entity in pairs(entities) do
             for _, component in pairs(entity.components) do
                 local callback = component.on_update
                 if callback then
@@ -80,5 +80,19 @@ return {
                 end
             end
         end
+    end,
+    render = function()
+        for _,entity in pairs(entities) do
+            for _, component in pairs(entity.components) do
+                local callback = component.on_render
+                if callback then
+                    local result, err = pcall(callback)
+                    if err then
+                        --// TODO: replace with error logging
+                        print(err)
+                    end
+                end
+            end
+        end
     end
 }