background: fix crash on reset

Fixes #219

Author: ammen99

src/background/background.cpp

@@ -321,6 +321,11 @@ WayfireBackground::WayfireBackground(WayfireShellApp *app, WayfireOutput *output
     });
 }
 
+WayfireBackground::~WayfireBackground()
+{
+    reset_background();
+}
+
 class WayfireBackgroundApp : public WayfireShellApp
 {
     std::map<WayfireOutput*, std::unique_ptr<WayfireBackground>> backgrounds;

src/background/background.hpp

@@ -71,4 +71,5 @@ class WayfireBackground
   public:
     WayfireBackground(WayfireShellApp *app, WayfireOutput *output);
     bool change_background();
+    ~WayfireBackground();
 };