VideoBackends: Support a different number of threads for precompiling

At runtime, we only really want a single shader compiler thread.
However, for initial boots, we can use a higher number to speed things
up.

Source/Core/VideoBackends/D3D/main.cpp

@@ -161,6 +161,7 @@ void VideoBackend::Video_Prepare()
   VertexShaderCache::Init();
   PixelShaderCache::Init();
   GeometryShaderCache::Init();
+  VertexShaderCache::WaitForBackgroundCompilesToComplete();
   D3D::InitUtils();
   BBox::Init();
 }