Optimize zone-zone checks in filler

Teardrops are calculated as zones, so when we create a board with
thousands of teardrops, we don't want to run O(n^2) checks on the
clearance between multiple teardrops.  Instead, we can check only the
clearance where the two zones have the potential to interact

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21226

pcbnew/zone_filler.cpp

@@ -1563,6 +1563,10 @@ void ZONE_FILLER::buildCopperItemClearances( const ZONE* aZone, PCB_LAYER_ID aLa
         if( checkForCancel( m_progressReporter ) )
             return;
 
+        // Only check zones whose bounding box overlaps the max clearance
+        if( !otherZone->GetBoundingBox().Intersects( zone_boundingbox ) )
+            continue;
+
         // Negative clearance permits zones to short
         if( evalRulesForItems( CLEARANCE_CONSTRAINT, aZone, otherZone, aLayer ) < 0 )
             continue;
@@ -1586,6 +1590,10 @@ void ZONE_FILLER::buildCopperItemClearances( const ZONE* aZone, PCB_LAYER_ID aLa
             if( checkForCancel( m_progressReporter ) )
                 return;
 
+            // Only check zones whose bounding box overlaps
+            if( !otherZone->GetBoundingBox().Intersects( zone_boundingbox ) )
+                continue;
+
             if( otherZone->GetIsRuleArea() )
             {
                 if( otherZone->GetDoNotAllowZoneFills() && !aZone->IsTeardropArea() )