Define DRAWABLE_UNDEFINED constant and display a warning when drawable is created with invalid flags.

Source/Urho3D/Graphics/Drawable.cpp

@@ -109,6 +109,10 @@ Drawable::Drawable(Context* context, unsigned char drawableFlags) :
     maxLights_(0),
     firstLight_(nullptr)
 {
+    if (drawableFlags == DRAWABLE_UNDEFINED || drawableFlags > DRAWABLE_ANY)
+    {
+        URHO3D_LOGERROR("Drawable with undefined drawableFlags");
+    }
 }
 
 Drawable::~Drawable()

Source/Urho3D/Graphics/Drawable.h

@@ -29,6 +29,7 @@
 namespace Urho3D
 {
 
+static const unsigned DRAWABLE_UNDEFINED = 0x0;
 static const unsigned DRAWABLE_GEOMETRY = 0x1;
 static const unsigned DRAWABLE_LIGHT = 0x2;
 static const unsigned DRAWABLE_ZONE = 0x4;