Migrate LAYER_TERRAIN* to a class

This is a very large one with heavy refactoring. The code was split in smaller
functions, boosting readability. Documentation was also improved.

No new functionality was added. The unused is_reversed flag was dropped; it was
not used in any known tileset and layer_order provides a better alternative.
The factorization makes it very straightforward to add more than 3 terrain
layers, limited only by the tileset reading code.

All shipped tilesets load correctly.

See #430.

client/CMakeLists.txt

@@ -60,6 +60,7 @@ add_library(
   layer_base_flags.cpp
   layer_darkness.cpp
   layer_special.cpp
+  layer_terrain.cpp
   luaconsole_common.cpp
   mapctrl_common.cpp
   mapview_common.cpp

client/layer.h

@@ -17,6 +17,7 @@ class QPixmap;
 
 struct city;
 struct player;
+struct terrain;
 struct tileset;
 struct unit;
 struct unit_type;
@@ -156,6 +157,14 @@ class layer {
    */
   virtual void free_player(int player_id) { Q_UNUSED(player_id); }
 
+  /**
+   * Initializes terrain-specific data.
+   */
+  virtual void initialize_terrain(const terrain *terrain)
+  {
+    Q_UNUSED(terrain);
+  }
+
   mapview_layer type() const { return m_layer; }
 
 protected: