Switch to single safe threaded model in TextureRD demo (#1038)

Multithreaded rendering is not safe in 4.2, so it should not be used
in our official demos for now.

compute/texture/project.godot

@@ -14,7 +14,3 @@ config/name="TestCustomTextures"
 run/main_scene="res://main.tscn"
 config/features=PackedStringArray("4.2")
 config/icon="res://icon.svg"
-
-[rendering]
-
-driver/threads/thread_model=2