There were actually two bugs. One was when the line that intersects itself
        the intersection point treated as it doesn't belong to the line.
        Second when edges partly coincide, wrong result produced when we try to find their
        intersection.


per-file comments:
  mysql-test/r/gis-precise.result
Fix for bug #804324 Assertion 0 in Gcalc_scan_iterator::pop_suitable_intersection
        test result updated.

  mysql-test/t/gis-precise.test
Fix for bug #804324 Assertion 0 in Gcalc_scan_iterator::pop_suitable_intersection
        test case added.

  sql/gcalc_slicescan.cc
Fix for bug #804324 Assertion 0 in Gcalc_scan_iterator::pop_suitable_intersection
        skip the intersection if it just line that intersects itself.

  sql/gcalc_tools.cc
Fix for bug #804324 Assertion 0 in Gcalc_scan_iterator::pop_suitable_intersection
        if edges coincide, just pick the first coinciding poing as an intersection.