Context2d: keep fillStyle/strokeStyle strong, fix resetState and ~Context2d teardown

[iurisilvio]

Context2d state-handling correctness (split out of #2582)

Three small, independent fixes to Context2d state handling. Split from #2582 — which now contains only the cairo_pattern_t refcounting — this PR is the correctness half and does not depend on it.

1. Keep fillStyle / strokeStyle as strong references

_fillStyle / _strokeStyle are weak references (.Reset(obj), refcount 0). For an image-backed CanvasPattern, the cairo pattern's surface points at the Image's pixel buffer (cairo_image_surface_create_for_data), which Image::clearData() frees unconditionally when the Image is GC'd. If the wrapper is collected while the Context2d state still references that pattern, drawing reads freed memory.

.Reset(obj, 1) keeps the wrapper alive while it is the active style. (Pairs with the source retention in #2583, which keeps the Image itself alive for the pattern's lifetime.)

2. resetState() leaves state dangling

void Context2d::resetState() {
  states.pop();      // pops one level only
  states.emplace();  // `state` still points at the popped/old top -> dangling
  ...
}

Fix: pop all states, emplace one, and re-point state = &states.top().

3. ~Context2d() teardown order

Pop states before destroying the cairo context, so each canvas_state_t is torn down while the context is still valid.

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Note: this overlaps with #2582 in SetFillStyle / SetStrokeStyle (both touch those lines). Whichever merges first, the other rebases trivially.