Add color aliasing (#1652)

You can refer to another color as a color value with $foo. You
can define any color you want with <color id="foo" value="#32323"/>.
Don't use circular definitinos since that will explode.

Addresses #1649

src/common/gui/SkinSupport.cpp

@@ -381,11 +381,14 @@ void Skin::reloadSkin(std::shared_ptr<SurgeBitmaps> bitmapStore)
             rgb = rgb >> 8;
 
             int r = rgb % 256;
-            colors[id] = VSTGUI::CColor(r, g, b);
+            colors[id] = ColorStore( VSTGUI::CColor(r, g, b) );
          }
-         else
+         else if( val[0] == '$' )
          {
-            colors[id] = VSTGUI::CColor(255, 0, 0);
+            colors[id] = ColorStore( val.c_str() + 1 );
+         }
+         else {
+            colors[id] = ColorStore( VSTGUI::CColor(255, 0, 0) );
          }
       }
    }
@@ -400,7 +403,16 @@ VSTGUI::CColor Skin::getColor(std::string id, const VSTGUI::CColor& def)
 {
    queried_colors.insert(id);
    if (colors.find(id) != colors.end())
-      return colors[id];
+   {
+      auto c = colors[id];
+      switch( c.type )
+      {
+      case ColorStore::COLOR:
+         return c.color;
+      case ColorStore::ALIAS:
+         return getColor( c.alias, def );
+      }
+   }
    return def;
 }
 } // namespace UI

src/common/gui/SkinSupport.h

@@ -84,7 +84,21 @@ class Skin
 private:
    std::vector<std::pair<std::string, props_t>> globals;
 
-   std::unordered_map<std::string, VSTGUI::CColor> colors;
+   struct ColorStore {
+      VSTGUI::CColor color;
+      std::string alias;
+
+      typedef enum {
+         COLOR,
+         ALIAS
+      } Type;
+
+      Type type;
+      ColorStore() : type( COLOR ), color( VSTGUI::kBlackCColor ) { }
+      ColorStore( VSTGUI::CColor c ) : type( COLOR ), color( c ) { }
+      ColorStore( std::string a ) : type( ALIAS ), alias( a ) { }
+   };
+   std::unordered_map<std::string, ColorStore> colors;
    std::unordered_set<std::string> queried_colors;
    std::unordered_map<std::string, int> imageIds;
    std::vector<Control> controls;