fix conditional pixel snapping for marker rasters

include/mapnik/marker_helpers.hpp

@@ -241,6 +241,8 @@ struct raster_markers_rasterizer_dispatch : mapnik::noncopyable
         {
             agg::rendering_buffer src_buffer((unsigned char *)src_.getBytes(),src_.width(),src_.height(),src_.width() * 4);
             pixfmt_pre pixf_mask(src_buffer);
+            if (snap_to_pixels_)
+            {
                 renb_.blend_from(pixf_mask,
                                  0,
                                  std::floor(marker_tr.tx + .5),
@@ -248,6 +250,15 @@ struct raster_markers_rasterizer_dispatch : mapnik::noncopyable
                                  unsigned(255*opacity));
             }
             else
+            {
+                renb_.blend_from(pixf_mask,
+                                 0,
+                                 marker_tr.tx,
+                                 marker_tr.ty,
+                                 unsigned(255*opacity));
+            }
+        }
+        else
         {
             using img_accessor_type = agg::image_accessor_clone<pixfmt_pre>;
             using interpolator_type = agg::span_interpolator_linear<>;