[Backport release-3_44] [3D] rubberband: avoid duplicates marker material #64445
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When a marker parameter is changed (for example enabling or disabling the markers, changing the color or the width), the `updateMarkerMaterial` material is called. This function always creates a new `QgsPoint3DBillboardMaterial` even if a material already exists. This entity ends up with multiple materials when it should only have one. This issue is fixed by checking if `mMarkerMaterial` already exists. If that's the case, the existing material is updated instead of creating a new one. If the markers are disabled, the material is removed from the entity as already done. Now, it also deleted and set to null to ensure its recreation if needed later on.
This ensures to avoid crashes if the entity is quickly deleted or updated. Indeed, Qt3D needs to keep both a frontend a backend in order to optimize the scene graph interactions. This can result in some entities being created and deleted in the frontend before the backend is able to handle those entities. Besides the parent logic is a mix of the Qt parent logic of any QObject and the parent/child logic of the scene graph. A simple rule to prevent any crashes is the following: - use a parent for entities in the constructor - do not use parents for components. An entity takes reponsability of of a component once added to the entity with the `addComponent` call. This rule is applied to the |mLineEntity` entity. This should help resolve segfaults that sometimes happen when using a rubberband.
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Backport #64269
Authored by: @ptitjano