Fixed segfault problem.  Added fast (but libgnomecanvas buggy) rendering.
Fixed segfault problem.  Added fast (but libgnomecanvas buggy) rendering.

When using polylines on the Gnome Canvas, you sometimes get the following
error message: 'attempt to put segment in horiz list twice'.  This is an
error in libgnomecanvas, because the polylines are valid.

The workaround is to plot out the polyline as a series of line segments.
The rendering in this case is slower.  This is not usually a problem, but
can become so for real-time displays (e.g., the plplotcanvas_animation).
So, a new function, gcw_use_fast_rendering(), and a new driver option
"-drvopt fast" have been added.

The default is to use the slow rendering, because this always works.
Fast rendering works most of the time, but sometimes produces error
messages and obviously wonky plots for 3D contour plots and vector plots.

A better solution will need to be found in collaboration with the gnome
team.

svn path=/trunk/; revision=6035

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot