Unify drawing of layers and interactions into "drawables".

src/fmri/Animation.hpp

@@ -2,31 +2,16 @@
 
 
 #include "utils.hpp"
+#include "Drawable.hpp"
 
 namespace fmri
 {
-    class Animation
+    class Animation : public Drawable
     {
     public:
         virtual ~Animation() = default;
 
-        virtual void draw(float step) = 0;
-
-    private:
-        static float getAlpha();
-
     protected:
-        template<typename It>
-        void patchTransparancy(It begin, It end)
-        {
-            if constexpr (std::tuple_size<Color>::value >= 4) {
-                const auto alpha = getAlpha();
-                for (; begin != end; ++begin) {
-                    Color &color = *begin;
-                    color[3] = alpha;
-                }
-            }
-        }
-
+        float getAlpha() override;
     };
 }