VideoCommon: add method to async shader compiler to clear pending/completed work (used on shutdown), this will in turn clear up any resources that the worker items may have held onto

Source/Core/VideoCommon/GraphicsModSystem/Runtime/CustomShaderCache.cpp

@@ -24,10 +24,16 @@ CustomShaderCache::CustomShaderCache()
 CustomShaderCache::~CustomShaderCache()
 {
   if (m_async_shader_compiler)
+  {
     m_async_shader_compiler->StopWorkerThreads();
+    m_async_shader_compiler->ClearAllWork();
+  }
 
   if (m_async_uber_shader_compiler)
+  {
     m_async_uber_shader_compiler->StopWorkerThreads();
+    m_async_uber_shader_compiler->ClearAllWork();
+  }
 }
 
 void CustomShaderCache::RetrieveAsyncShaders()