[layer tree] Avoid needless layer tree model computations for scale-based layer visibility

[nirvn]

Description

This PR optimizes the layer tree model by avoiding needless computations around scale-based layer visibility. Optimizations include:

• If the map canvas scale hasn't changed, avoid triggering scale-based layer visibility check loop
• In cases scale has changed, make sure the model only emits dataChanged for layers that go from hidden to visible (or vice versa)
• Restrict dataChanged roles to only the ones that are impacted by visibility change

With the PR applied, I can't see any freeze when panning on the canvas with the test project in #38978 .

[m-kuhn]

Works and code looks good