[GPU] Async shader compilation for D3D12 and Vulkan

Adds async_shader_compilation cvar (default true) that forces new
pipelines to be created async from the main thread, skips draws entirely
on D3D12 and on vulkan replaces pixel shader with placeholder until the
real one is ready. Causes some visual artifacts on first load but
greatly reduces load times and stutter.

src/xenia/gpu/d3d12/d3d12_command_processor.cc

@@ -2583,6 +2583,15 @@ bool D3D12CommandProcessor::IssueDraw(xenos::PrimitiveType primitive_type,
     return false;
   }
 
+  if (cvars::async_shader_compilation) {
+    if (pipeline_cache_->GetD3D12PipelineByHandle(pipeline_handle) == nullptr) {
+      XELOGI("Skipping draw - pipeline not ready");
+      return true;
+    }
+    // Re-fetch root signature now that pipeline is ready.
+    root_signature = pipeline_cache_->GetRootSignatureByHandle(pipeline_handle);
+  }
+
   // Update the textures - this may bind pipelines.
   uint32_t used_texture_mask =
       vertex_shader->GetUsedTextureMaskAfterTranslation() |