Properly set trailing edge major of B span

The B span isn't processed further for the current block, so it was
phasing through tiles on the next z-level without checking them.

src/shadowcasting.cpp

@@ -72,7 +72,7 @@ struct span {
 template<typename T, bool( *is_transparent )( const T &, const T & ), T( *accumulate )( const T &, const T &, const int & )>
 static void split_span( std::list<span<T>> &spans, typename std::list<span<T>>::iterator &this_span,
                         T &current_transparency, const T &new_transparency, const T &last_intensity,
-                        const int distance, slope &new_start_minor,
+                        const tripoint &delta, const int distance, slope &new_start_minor,
                         const slope &trailing_edge_major, const slope &leading_edge_major,
                         const slope &trailing_edge_minor, const slope &leading_edge_minor )
 {
@@ -114,9 +114,10 @@ static void split_span( std::list<span<T>> &spans, typename std::list<span<T>>::
 
         // Emit the B span if present, placing it before the current span in the list
         if( trailing_edge_minor > this_span->start_minor ) {
+            const slope next_trailing_edge_major( ( delta.z + 1 ) * 2 - 1, delta.y * 2 + 1 );
             spans.emplace( this_span,
                            std::max( this_span->start_major, trailing_edge_major ),
-                           std::min( this_span->end_major, leading_edge_major ),
+                           std::min( this_span->end_major, next_trailing_edge_major ),
                            this_span->start_minor, trailing_edge_minor,
                            next_cumulative_transparency );
         }
@@ -298,7 +299,7 @@ void cast_horizontal_zlight_segment(
                     // Handle spliting the span into up to 4 separate spans
                     split_span<T, is_transparent, accumulate>( spans, this_span, current_transparency,
                             new_transparency, last_intensity,
-                            distance, new_start_minor,
+                            delta, distance, new_start_minor,
                             trailing_edge_major, leading_edge_major,
                             trailing_edge_minor, leading_edge_minor );
                 }
@@ -480,7 +481,7 @@ void cast_vertical_zlight_segment(
                     // Handle spliting the span into up to 4 separate spans
                     split_span<T, is_transparent, accumulate>( spans, this_span, current_transparency,
                             new_transparency, last_intensity,
-                            distance, new_start_minor,
+                            delta, distance, new_start_minor,
                             trailing_edge_major, leading_edge_major,
                             trailing_edge_minor, leading_edge_minor );
                 }