raster: base draw_mp_circle() on plotCircle() from

https://zingl.github.io/bresenham.c by Zingl Alois rather than
  RosettaCode one, better at small scales plus simplifies
  licence/attribution issues;
  fix licence/attribution issues for `plot_hexline()` also which
  is actually based on `plotLine()` in zingl code (ticket #357,
  props stefan6419846)
debian/copyright: update with above

debian/copyright

@@ -115,6 +115,13 @@ Copyright:
     Copyright (C) 2004 (Grandzebu).
 License: BSD-3-Clause and GPL-2.0-or-later
 
+Files:
+    backend/raster.c
+Copyright:
+    Copyright (C) 2008-2026 Robin Stuart <rstuart114@gmail.com>
+    Copyright (c) 2020 zingl
+License: BSD-3-Clause and MIT
+
 Files:
     docs/lua-crossrefs/lua-crossrefs.lua
     getopt/*