Add helper functions to the base layer class

They correspond to two variables declared at the beginning of
fill_sprite_array(), do_draw_unit and solid_bg, that are used by several layer
types.

See #430.

client/layer.cpp

@@ -11,6 +11,7 @@
     \_____/ /                     If not, see https://www.gnu.org/licenses/.
       \____/        ********************************************************/
 
+#include "control.h"
 #include "tilespec.h"
 
 #include "layer.h"
@@ -35,4 +36,58 @@ layer::fill_sprite_array(const tile *ptile, const tile_edge *pedge,
                              pcity, putype);
 }
 
+/**
+ * @brief Whether a unit should be drawn.
+ * @param ptile The tile where to draw (can be null)
+ * @param punit The unit that should be drawn (can be null)
+ */
+bool layer::do_draw_unit(const tile *ptile, const unit *punit) const
+{
+  if (!punit) {
+    // Can't draw a non-existing unit.
+    return false;
+  }
+
+  // There is a unit.
+
+  if (!ptile) {
+    // No tile (so not on the map) => always draw.
+    return true;
+  }
+
+  // Handle options to turn off drawing units.
+  if (gui_options.draw_units) {
+    return true;
+  } else if (gui_options.draw_focus_unit && unit_is_in_focus(punit)) {
+    return true;
+  } else {
+    return false;
+  }
+}
+
+/**
+ * @brief Whether a solid background should be drawn on a tile instead of its
+ *        terrain.
+ *
+ * Query this function to know whether you should refrain from drawing
+ * "terrain-like" sprites (terrain, darkness, water follow this setting).
+ *
+ * @returns `true` when the solid background is enabled and a unit or city is
+ *          drawn on the tile.
+ *
+ * @param ptile The tile where to draw (can be null)
+ * @param punit The unit that could be drawn (can be null)
+ * @param pcity The city that could be drawn (can be null)
+ */
+bool layer::solid_background(const tile *ptile, const unit *punit,
+                             const city *pcity) const
+{
+  if (!gui_options.solid_color_behind_units) {
+    // Solid background turned off (the default).
+    return false;
+  }
+
+  return do_draw_unit(ptile, punit) || (gui_options.draw_cities && pcity);
+}
+
 } // namespace freeciv

client/layer.h

@@ -161,6 +161,10 @@ class layer {
 protected:
   struct tileset *tileset() const { return m_ts; }
 
+  bool do_draw_unit(const tile *ptile, const unit *punit) const;
+  bool solid_background(const tile *ptile, const unit *punit,
+                        const city *pcity) const;
+
 private:
   struct tileset *m_ts;
   mapview_layer m_layer;

client/layer_darkness.cpp

@@ -41,13 +41,7 @@ std::vector<drawn_sprite> layer_darkness::fill_sprite_array(
   }
 
   // Don't draw darkness when the solid background is used
-  bool do_draw_unit =
-      (punit
-       && (gui_options.draw_units || !ptile
-           || (gui_options.draw_focus_unit && unit_is_in_focus(punit))));
-
-  if (!(gui_options.solid_color_behind_units
-        && (do_draw_unit || (pcity && gui_options.draw_cities)))) {
+  if (!solid_background(ptile, punit, pcity)) {
     return {};
   }