`Container::background` now stores `Option<Paint>` instead of `Option<Color>`, and the builder accepts `impl Into<Paint>` so callers can pass a plain `Color` (auto-wrapped in `Paint::Solid` via the trait impl) or an explicit `LinearGradient` / `RadialGradient`. `layout_and_draw` switches from `canvas.fill_rect( rect, bg, corners )` to `canvas.fill_paint_rect( rect, &bg, corners )` to consume the wider type.
No behaviour change for existing call sites — solid-colour containers keep working unchanged thanks to the `Into<Paint>` for `Color`.
Previously only `set_min_size()` was called with the requested size, on the assumption that the compositor would adopt it as the initial dimension. In practice that doesn't hold: xdg-shell has no "preferred initial size" primitive and compositors are free to pick any size within `[min, max]` on the first configure. The window ended up opening at the 800x600 fallback instead of the size the application had asked for.
The fix pins `min == max` before the first commit, which forces the compositor to honour the requested size in its first configure, and then releases `max_size` from the configure handler via the `pending_size_hint_unpin` latch so the surface remains user-resizable afterwards. The 800x600 fallback now only applies when the application does not provide a hint.
