Patch from Mark Wooding to introduce a draw_thick_line() function in

Patch from Mark Wooding to introduce a draw_thick_line() function in
the drawing API, for use by Loopy. It's optional: drawing.c will
construct an acceptable alternative using a filled polygon if the
front end doesn't provide it.

Net and Netslide previously had static functions called
draw_thick_line(), whose claim to the name is less justified and so
they've been renamed.

[originally from svn r8962]