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;

src/qt/qt_platform.cpp

@@ -54,7 +54,6 @@ QElapsedTimer elapsed_timer;
 
 static std::atomic_int blitmx_contention = 0;
 static std::recursive_mutex blitmx;
-static thread_local std::unique_lock blit_lock { blitmx, std::defer_lock };
 
 class CharPointer {
 public:
@@ -469,17 +468,17 @@ void dynld_close(void *handle)
 void startblit()
 {
     blitmx_contention++;
-    if (blit_lock.try_lock()) {
+    if (blitmx.try_lock()) {
         return;
     }
 
-    blit_lock.lock();
+    blitmx.lock();
 }
 
 void endblit()
 {
     blitmx_contention--;
-    blit_lock.unlock();
+    blitmx.unlock();
     if (blitmx_contention > 0) {
         // a deadlock has been observed on linux when toggling via video_toggle_option
         // because the mutex is typically unfair on linux