Implement BG callback

SMILEBASIC/VM.d

@@ -2735,11 +2735,22 @@ class CallBG : Code
 {
     override void execute(VM vm)
     {
+        vm.petitcomputer.callidx++;
         if (!vm.petitcomputer.callback)
         {
-            vm.petitcomputer.callidx++;
             return;
         }
-        vm.petitcomputer.callidx++;
+        if (!vm.petitcomputer.isValidLayer(vm.petitcomputer.callidx))
+        {
+            vm.petitcomputer.callback = false;
+            return;
+        }
+        auto callback = vm.petitcomputer.getBG(vm.petitcomputer.callidx).callback;
+        vm.pc--;
+        if (!callback.isCallable)
+        {
+            return;
+        }
+        callback(vm);
     }
 }

SMILEBASIC/bg.d

@@ -249,4 +249,6 @@ class BG
                 }
             }
         }
+    import otya.smilebasic.vm;
+    Callback callback;
 }

SMILEBASIC/builtinfunctions.d

@@ -2223,6 +2223,17 @@ class BuiltinFunction
         }
         BGLOAD(p, layer, 0, 0, p.getBG(layer).width, p.getBG(layer).height, ary, 0);
     }
+    static void BGFUNC(PetitComputer p, int layer, wstring func)
+    {
+        if (!p.isValidLayer(layer))
+        {
+            throw new OutOfRange("BGFUNC", 1);
+        }
+        auto callback = p.vm.createCallback(func);
+        if (callback.type == CallbackType.none)
+            throw new IllegalFunctionCall("BGUNC"/*, 2*/);
+        p.getBG(layer).callback = callback;
+    }
     static void EFCON()
     {
     }

SMILEBASIC/petitcomputer.d

@@ -526,6 +526,16 @@ class PetitComputer
         SDL_SetWindowSize(window, cast(int)(currentDisplay.windowSize.width * scaleX), cast(int)(currentDisplay.windowSize.height * scaleY));
     }
 
+    bool isValidLayer(int layer)
+    {
+        if (layer < 0)
+            return false;
+        if(displaynum)
+        {
+            return layer < bg.length - bgmax;
+        }
+        return layer < bgmax;
+    }
     BG getBG(int layer)
     {
         if(displaynum)