GPU: Metal: Fix background render crash

GHOST_CreateSystemBackground was not being
followed by the now mandatory GPU_backend_ghost_system_set

source/blender/gpu/intern/gpu_shader_builder.cc

@@ -46,6 +46,7 @@ void ShaderBuilder::init_system()
 {
   CLG_init();
   ghost_system_ = GHOST_CreateSystemBackground();
+  GPU_backend_ghost_system_set(ghost_system_);
 }
 
 bool ShaderBuilder::init_context()

source/blender/gpu/opengl/gl_compilation_subprocess.cc

@@ -167,6 +167,7 @@ void GPU_compilation_subprocess_run(const char *subprocess_name)
 
   GHOST_SystemHandle ghost_system = GHOST_CreateSystemBackground();
   BLI_assert(ghost_system);
+  GPU_backend_ghost_system_set(ghost_system);
   GHOST_GPUSettings gpu_settings = {0};
   gpu_settings.context_type = GHOST_kDrawingContextTypeOpenGL;
   GHOST_ContextHandle ghost_context = GHOST_CreateGPUContext(ghost_system, gpu_settings);

source/blender/windowmanager/intern/wm_window.cc

@@ -1930,6 +1930,7 @@ void wm_ghost_init_background()
   GHOST_SetBacktraceHandler((GHOST_TBacktraceFn)BLI_system_backtrace);
 
   g_system = GHOST_CreateSystemBackground();
+  GPU_backend_ghost_system_set(g_system);
 
   GHOST_Debug debug = {0};
   if (G.debug & G_DEBUG_GHOST) {