[task #13658] Work on concave polygons too

[Sachin Kumar Singh]

Follow-up Comment #16, task #13658 (project gnuastro):

I've tried many points around the problematic region in my
`gal_concavesort_isinside_polygon' but there doesn't seem to be any problem at
all. But when reproducing the steps I see that the erroneous region still
appears. I even applied a rounding error constant as used in `polygon.c`, but
the error still persists. I tried a few other points and then there seemed a
pattern. 

Firstly, the erroneous region is detached from the main polygon(in ds9 it
shows a blank pixel value at the apparent joining). I think it is independent
of the main polygon.

If the point is outside(as shown in the figure) the intersection of the 2
points is also considered for the cropping region.

Hence I think the problem arises while performing the actual clipping because,
during a convex polygon clipping, no extra region is clipped but during a
concave clipping errors might arise(if using Sutherland–Hodgman algorithm
<https://en.wikipedia.org/wiki/Sutherland%E2%80%93Hodgman_algorithm>).

Which algorithm is used in onecrop.c to clip polygons? How to fix if it used a
more general clipping algorithm because `gal_concavesort_isinside_polygon' is
tested is not showing any problems?



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