gdi32/dibdrv: don't add_join if points are coincident in wide_line_segments (try 3)

[Daniel Lehman]

try 2 - integrate into existing tests; updated commit message
try 3 - change test to use straight lines instead of diagonals.  PolyPolyline draws diagonal lines slightly differently from Windows and was producing different hashes (although visually the output looks the same).  The point of the patch is to fix a crash given empty segments, not really to test drawing output
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