Scrollbar: Fix ScrollbarX enable test subtracting WindowPadding.x. This has been here since 65a191c (#246) tho at the time it is possible that SizeContents wasn't calculated as evenly.

Test: auto-resize with: ImGui::Begin("Test", NULL, ImGuiWindowFlags_HorizontalScrollbar); ImGui::Button("##xxx", ImVec2(500,500)); ImGui::End();