Revert "Fix crash at exit due to a unreleased mutex."

This reverts commit 80e5470006.

std::unique_lock is incapable of recursively locking a mutex, which is needed for multi-monitor setups.
As a result it will crash/show undefined behaviour when switching renderers. Switch to instead calling
endblit() after pc_close to avoid crashes; at this point the CPU thread is now terminated so the mutex
no longer remains held by it.

src/qt/qt_main.cpp

@@ -289,6 +289,7 @@ int main(int argc, char* argv[]) {
     cpu_thread_run = 0;
     main_thread->join();
     pc_close(nullptr);
+    endblit();
 
     socket.close();
     return ret;