PR#581 Signal indicator does not show if ribi arrow is missing

[Ranran]

As the title says, this means the loss of this feature in the standard's pakset and the old extended(experimental) pakset.

EDIT:
Which solution do you guys think is better?
(1) Show this with signal resevation as well as standard.
However, I have seen requests to separate those display options for this implementation of the standard.
Players may only want to check one or the other.
(2) Ignore missing ribi arrow
(3) Assign this function to a new key

EDIT2:
Pull request #581 fixes this with method (2). This is the least impactful and simplest implementation.

[Ranran]

The bugfix patch metamorphose.
If the pakset doesn't have a ribi_arrow, show a white arrow instead. This provides a better look than ribi_arrow and may eliminate some issues (night mode and diagonal and slope look). However, note that the arrows still do not appear in the road construction preview.

[jamespetts]

Thank you very much for this: now incorporated.