Enforce thread safety in clearance and creepage checks

Previously, these checks injected a custom handler to add graphic
objects to a DRC marker. This was not thread-safe and was causing
non-deterministic crashes. The DRC reporting methods now accept
a customer handler which is called on the newly created
PCB_MARKER within the commit context. This defaults to nullptr
for DRC checks which do not require graphics or other additional
processing.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19282

pcbnew/python/scripting/pcbnew_scripting_helpers.cpp

@@ -609,7 +609,8 @@ bool WriteDRCReport( BOARD* aBoard, const wxString& aFileName, EDA_UNITS aUnits,
     engine->SetProgressReporter( nullptr );
 
     engine->SetViolationHandler(
-            [&]( const std::shared_ptr<DRC_ITEM>& aItem, VECTOR2D aPos, int aLayer )
+            [&]( const std::shared_ptr<DRC_ITEM>& aItem, VECTOR2D aPos, int aLayer,
+                 DRC_CUSTOM_MARKER_HANDLER* aCustomHandler )
             {
                 if( aItem->GetErrorCode() == DRCE_MISSING_FOOTPRINT
                     || aItem->GetErrorCode() == DRCE_DUPLICATE_FOOTPRINT