Add doc::find_layer_index()

It'll be useful to known the index of a layer inside a list (e.g. a list
of all layers, or a list of all browsable layers, etc.).

src/doc/CMakeLists.txt

@@ -44,6 +44,7 @@ add_library(doc-lib
   layer.cpp
   layer_index.cpp
   layer_io.cpp
+  layer_list.cpp
   mask.cpp
   mask_boundaries.cpp
   mask_io.cpp

src/doc/layer_list.cpp

@@ -0,0 +1,27 @@
+// Aseprite Document Library
+// Copyright (c) 2016 David Capello
+//
+// This file is released under the terms of the MIT license.
+// Read LICENSE.txt for more information.
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "doc/layer.h"
+#include "doc/layer_list.h"
+
+#include <algorithm>
+
+namespace doc {
+
+layer_t find_layer_index(const LayerList& layers, const Layer* layer)
+{
+  auto it = std::find(layers.begin(), layers.end(), layer);
+  if (it != layers.end())
+    return it - layers.begin();
+  else
+    return 0;
+}
+
+} // namespace doc

src/doc/layer_list.h

@@ -17,6 +17,8 @@ namespace doc {
   typedef std::vector<Layer*> LayerList;
   typedef int layer_t;
 
+  layer_t find_layer_index(const LayerList& layers, const Layer* layer);
+
 } // namespace doc
 
 #endif  // DOC_LAYER_LIST_H_INCLUDED