shell-global: Only set the scale factor if get_setting succeeded

If gdk_screen_get_setting fails, like if it's running without XSettings,
then the GValue will have a value of 0. A lot of code tries to divide by
the scale factor. This produces NaN, and combined with the fact that NaN
is "leaky", we very quickly end up spinning out of control.
shell-util: Fix warning in shell_util_cursor_tracker_to_clutter

If the sprite is NULL, like if a Wayland app wanted to hide the cursor,
then we need to hide the ClutterTexture on our side, as ClutterTexture
has no easy way to tell it to paint nothing.