Direction neutral paragraph support for Fribidi

[niello]

Fix #163.
Only for Fribidi currently. Someone who knows SheenBidi could make the same change there.

[khaledhosny]

I’m adding a test for this, but I’m not sure this fix is enough. With this fix resolved dir will be RAQM_DIRECTION_DEFAULT but FriBiDi would still treat neutral characters as LTR. The resolved dir is supposed to communicate to the client what direction the text will use when no paragraph direction was explicitly set, so in this case it should be RAQM_DIRECTION_LTR. Resolved direction really needs to be either LTR or RTL, this is how the code always used it internally.

I think we need a different way to communicate to the client that the text is all neutral characters and no paragraph direction can be detected.

[khaledhosny]

The current code is wrong of course and need to be fixed regardless of this.

[khaledhosny]

#175 fixes the bad resolved dir, for the use case here, I suggest we add an API to get the natural paragraph direction regardless of the set one.

[niello]

Sounds good, that API would help