Patch from James H providing lots more paranoid casting. Also one

actual behaviour change: Untangle now permits dragging with the
right mouse button, which has exactly the same effect as it does
with the left. (Harmless on desktop platforms, but helpful when
"right-click" is achieved by press-and-hold; now the drag takes
place even if you hesitate first.)

[originally from svn r8177]

drawing.c

@@ -281,5 +281,5 @@ void print_line_width(drawing *dr, int width)
      * _square root_ of the main puzzle scale. Double the puzzle
      * size, and the line width multiplies by 1.4.
      */
-    dr->api->line_width(dr->handle, sqrt(dr->scale) * width);
+    dr->api->line_width(dr->handle, (float)sqrt(dr->scale) * width);
 }