Fix a couple of potential crashes on exit.

- Font cache shutdown explicitly
- Pending GUI snapshot cleared explicitly

Both of these are dependencies on the osgViewer / GLContext.

src/GUI/FGFontCache.cxx

@@ -85,6 +85,11 @@ FGFontCache* FGFontCache::instance()
     return global_fontCacheInstance.get();
 }
 
+void FGFontCache::shutdown()
+{
+    global_fontCacheInstance.reset();
+}
+
 FGFontCache::FGFontCache() :
     _initialized(false)
 {

src/GUI/FGFontCache.hxx

@@ -78,6 +78,7 @@ private:
 public:
     // note this accesor is NOT thread-safe
     static FGFontCache* instance();
+    static void shutdown();
 
     ~FGFontCache();
 

src/GUI/gui.h

@@ -47,6 +47,8 @@ extern void guiErrorMessage(const char *txt);
 extern void guiErrorMessage(const char *txt, const sg_throwable &throwable);
 
 extern bool fgDumpSnapShot();
+void fgCancelSnapShot();
+
 extern void fgDumpSceneGraph();
 extern void fgDumpTerrainBranch();
 extern void fgPrintVisibleSceneInfoCommand();

src/GUI/gui_funcs.cxx

@@ -483,6 +483,11 @@ namespace
             return true;
         }
 
+        static void cancel()
+        {
+            _snapShotOp = nullptr;
+        }
+
     private:
         // constructor to be executed in main loop's thread
         GUISnapShotOperation() :
@@ -567,6 +572,11 @@ bool fgDumpSnapShot ()
     return GUISnapShotOperation::start();
 }
 
+void fgCancelSnapShot()
+{
+    GUISnapShotOperation::cancel();
+}
+
 // do an entire scenegraph dump
 void fgDumpSceneGraph()
 {

src/Main/globals.cxx

@@ -54,6 +54,7 @@
 #include <Navaids/navlist.hxx>
 #include <Viewer/renderer.hxx>
 #include <Viewer/viewmgr.hxx>
+#include <GUI/FGFontCache.hxx>
 
 #include "globals.hxx"
 #include "locale.hxx"
@@ -223,6 +224,9 @@ FGGlobals::~FGGlobals()
     // renderer touches subsystems during its destruction
     set_renderer(NULL);
 
+    FGFontCache::shutdown();
+    fgCancelSnapShot();
+
     delete subsystem_mgr;
     subsystem_mgr = NULL; // important so ::get_subsystem returns NULL
     vw = 0; // don't delete the viewer until now