[task #13658] Work on concave polygons too

[Sachin Kumar Singh]

Follow-up Comment #14, task #13658 (project gnuastro):

I've added a new function inside the concavesort.c which uses winding number
algorithm
<https://en.wikipedia.org/wiki/Point_in_polygon#Winding_number_algorithm> to
identify points inside a non-convex polygon. 
[https://github.com/sachinkumarsingh092/gnuastro/tree/task-concave-sort]

I've also removed the global variables which really were a nuisance and
instead used pointers:-). Some minor bugs were fixed and better names are
assigned to the functions. This also fixes task #14639 to a certain extent,
only proper sanity checks and formatting for the SAO DS9 format remains.  

Points are now sorted in the anti-clockwise direction. 
Regarding the option to avoid sorting and let the program use the same order
that the user-provided, I think we can take the input as an edge list and use
a flag to indicate that the user is giving such an input. Then we can use a
different sort function to sort the edges instead of points(which will be
easier to implement using a map/tree data structure). 

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