Fix T99706: Crash rendering with headless builds

When rendering with headless builds, show an error instead of crashing.

Previously GPU_backend_init was called indirectly from
DRW_opengl_context_create, a new function is now called from the window
manager (GPU_backend_init_once), so it's possible to check if the GPU
has a back-end.

This also disables the `bgl` Python module when building WITH_HEADLESS.

Reviewed By: fclem

Ref D15463

source/blender/windowmanager/intern/wm_init_exit.c

@@ -169,6 +169,10 @@ void WM_init_opengl(void)
     wm_ghost_init(NULL);
   }
 
+  if (!GPU_backend_init_once()) {
+    return;
+  }
+
   /* Needs to be first to have an OpenGL context bound. */
   DRW_opengl_context_create();