Bugfix: clean up static resources correctly

93924af160 might corrupt memory with gcc-4.6.1 when
finishing fluxbox (clicking 'exit', sending it a SIGINT). Allthough the order, in which static / global
objects are initialized is undefined (at least between separate compilation units), the order in
which they are destroyed is well defined: in reverse order of initialization.

this means, that if 'ScreenImlibContextContainer contexts' (of ImageImlib2.cc)
gets initialized AFTER 'ImageImlib2 imlib2_loader' of Image.cc, it gets destroyed before
imlib2_loader. When that happens, ~ImageImlib2() works on a destroyed object.

(That lead to '* glibc detected * fluxbox: corrupted double-linked list: 0x0000000000dd2710 ***'
later on in 'iconv_close')

src/FbTk/ImageImlib2.cc

@@ -30,11 +30,20 @@
 
 namespace {
 
-typedef std::map<int, Imlib_Context> ScreenImlibContextContainer;
+class ScreenImlibContextContainer : public std::map<int, Imlib_Context> {
+public:
+    ~ScreenImlibContextContainer() {
+
+        std::map<int, Imlib_Context>::iterator it = this->begin();
+        std::map<int, Imlib_Context>::iterator it_end = this->end();
+        for (; it != it_end; it++) {
+            imlib_context_free(it->second);
+        }
+    }
+};
 typedef ScreenImlibContextContainer::iterator ScreenImlibContext;
 
 ScreenImlibContextContainer contexts;
-
 } // anon namespace
 
 
@@ -69,16 +78,6 @@ ImageImlib2::ImageImlib2() {
     }
 }
 
-ImageImlib2::~ImageImlib2() {
-    
-    ScreenImlibContext it = contexts.begin();
-    ScreenImlibContext it_end = contexts.end();
-    for (; it != it_end; it++) {
-        imlib_context_free(it->second);
-    }
-    contexts.clear();
-}
-
 PixmapWithMask *ImageImlib2::load(const std::string &filename, int screen_num) const {
 
     Display *dpy = FbTk::App::instance()->display();

src/FbTk/ImageImlib2.hh

@@ -28,7 +28,6 @@ namespace FbTk {
 class ImageImlib2: public ImageBase {
 public:
     ImageImlib2();
-    ~ImageImlib2();
     PixmapWithMask *load(const std::string &filename, int screen_num) const;
 };