Add process for programs to voluntarily remove renderdoc. Refs #115

* To remove renderdoc (e.g. if the API version doesn't match what is
  expected), user code can call RENDERDOC_Shutdown() to remove the running
  thread and undo any library hooks, then unload the dynamic library.
* This still isn't recommended as there could be unexpected side-effects,
  and it will definitely break if this happens after any APIs have been
  initialised, but it's an option.

renderdoc/replay/entry_points.cpp

@@ -27,6 +27,7 @@
 #include "maths/formatpacking.h"
 #include "serialise/serialiser.h"
 #include "core/core.h"
+#include "hooks/hooks.h"
 #include "replay/replay_renderer.h"
 #include "api/replay/renderdoc_replay.h"
 
@@ -90,6 +91,13 @@ int RENDERDOC_CC RENDERDOC_GetAPIVersion()
 	return RENDERDOC_API_VERSION;
 }
 
+extern "C" RENDERDOC_API
+void RENDERDOC_CC RENDERDOC_Shutdown()
+{
+	RenderDoc::Inst().Shutdown();
+	LibraryHooks::GetInstance().RemoveHooks();
+}
+
 extern "C" RENDERDOC_API
 void RENDERDOC_CC RENDERDOC_LogText(const char *text)
 {