Columns: avoid growing ColumnsData vector when closing a window with no columns (#125)

(Will break calling GetColumnOffset() when there's no column but it
would only have worked after the first frame anyway, so basically it
didn't work)

imgui.cpp

@@ -8910,9 +8910,7 @@ void ImGui::Columns(int columns_count, const char* id, bool border)
     }
     else
     {
-        window->DC.ColumnsData.resize(2);
-        window->DC.ColumnsData[0].OffsetNorm = 0.0f;
-        window->DC.ColumnsData[1].OffsetNorm = 1.0f;
+        window->DC.ColumnsData.resize(0);
     }
 }