Bug 1540581 - P6. Tidy some C++ declarations in gfx/. r=gerald,jrmuizel

* Remove redundant virtual keywords
* Mark all destructors of inheriting classes as virtual for clarity
* Mark all classes without virtual destructor as final (exposed errors)
* Make destructor virtual where it needed to be (some were missing)
* Replace empty ({}) code declaration in header with = default
* Remove virtual unused methods

I probably missed some, it quickly became a rabbit hole.

Differential Revision: https://phabricator.services.mozilla.com/D26060

gfx/2d/JobScheduler_win32.cpp

@@ -22,7 +22,7 @@ class WorkerThreadWin32 : public WorkerThread {
                              static_cast<WorkerThread*>(this), 0, nullptr);
   }
 
-  ~WorkerThreadWin32() {
+  virtual ~WorkerThreadWin32() {
     ::WaitForSingleObject(mThread, INFINITE);
     ::CloseHandle(mThread);
   }