[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/gpu_flags.h

@@ -36,6 +36,8 @@ DECLARE_bool(disassemble_pm4);
 
 DECLARE_bool(no_discard_stencil_in_transfer_pipelines);
 
+DECLARE_bool(async_shader_compilation);
+
 DECLARE_bool(gpu_3d_to_2d_texture);
 
 #define XE_GPU_FINE_GRAINED_DRAW_SCOPES 1