[Masking] Fix copy/paste of symbol or converting renderer

[troopa81]

Properly reset symbol layer ids when copy/pasting symbol or when converting a rule based renderer into a categorized renderer.

Fixes #46402 where issues comes from symbol layers having identical symbol layers ids, coming from I presume, actions described just above

[github-actions]

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[nyalldawson]

I'd lean toward a dynamic_cast to QgsMaskMarkerSymbolLayer here and then directly calling QgsMaskMarkerSymbolLayer::clearMasks to avoid the virtual clearMasks method in QgsSymbolLayer.

If later on we find other symbol layer types which need this handled then we can safely resurrect the virtual method without any API breakage.

[troopa81]

I considered this solution but preferred to have clearMasks() method for 2 reasons:

• to be consistent with masks() method (this is not the case of setMasks() but maybe this one should also be defined as a virtual method of QgsSymbolLayer)
• dynamic_cast is a bad design and should be avoided

Even if we never get a symbol layer types implementing masks, why do you think it is better with dynamic_cast ?

[nyalldawson]

@troopa81

why do you think it is better with dynamic_cast ?

It keeps the base class API smaller and avoids cluttering it with methods which have no use for all but one subclass. And in this case the dynamic_cast is only in one place in a static function from a utils class, so I think it's fine for that to have the little bit of added complexity.