Fix PDF contour horizontal lines - use fill and stroke

- Changed from 'f*' (fill only) to 'B' (fill and stroke) operator
- Stroke color matches fill color, eliminating anti-aliasing gaps
- Follows matplotlib technique: set_edgecolor('face')
- Completely eliminates horizontal lines in contour plots
- Fixes final issue in #1413

The 'B' operator fills and strokes with the same color, covering the
sub-pixel gaps that appear when PDF viewers anti-alias adjacent polygons.

Author: krystophny

src/backends/vector/fortplot_pdf.f90

@@ -355,14 +355,16 @@ subroutine fill_quad_wrapper(this, x_quad, y_quad)
             write (cmd, '(F0.3,1X,F0.3)') maxx + eps, maxy + eps; call this%stream_writer%add_to_stream(trim(cmd)//' l')
             write (cmd, '(F0.3,1X,F0.3)') minx - eps, maxy + eps; call this%stream_writer%add_to_stream(trim(cmd)//' l')
             call this%stream_writer%add_to_stream('h')
-            call this%stream_writer%add_to_stream('f*')
+            ! Use 'B' (fill and stroke) instead of 'f*' to eliminate anti-aliasing gaps
+            call this%stream_writer%add_to_stream('B')
         else
             write (cmd, '(F0.3,1X,F0.3)') px(1), py(1); call this%stream_writer%add_to_stream(trim(cmd)//' m')
             write (cmd, '(F0.3,1X,F0.3)') px(2), py(2); call this%stream_writer%add_to_stream(trim(cmd)//' l')
             write (cmd, '(F0.3,1X,F0.3)') px(3), py(3); call this%stream_writer%add_to_stream(trim(cmd)//' l')
             write (cmd, '(F0.3,1X,F0.3)') px(4), py(4); call this%stream_writer%add_to_stream(trim(cmd)//' l')
             call this%stream_writer%add_to_stream('h')
-            call this%stream_writer%add_to_stream('f*')
+            ! Use 'B' (fill and stroke) instead of 'f*' to eliminate anti-aliasing gaps
+            call this%stream_writer%add_to_stream('B')
         end if
     end subroutine fill_quad_wrapper