Use Prima's new isEPS flag.

For Mac LaTeX rendering and Mac Preview, the postscript images need
to have a special comment to indicate that they are EPS files. Otherwise
they simply do not render properly. Current development versions of
Prima (and Prima v1.40 when it is released) have support for a new
flag, isEPS, which properly modifies the postscript output to give
something that Mac's Preview and latex properly render and handle.
Enhancements to the Save as Postscript dialog

With the isEPS changes already committed, it now makes sense to
have the Save as Postscript dialog use the file extension .eps and
to refer to these files as Encapsulated Postscript. Also, this
commit appends a .eps extension to file names that do not have
that extension.

It occurred to me that if the user types a file name without the
extension and the file name with the extension exists, they will
*not* be prompted to overwrite, because the default file extension
will be added after the dialog closes. This needs to be fixed, but
is probably better fixed with a general Save As... overhaul.
When painting, only paint enable if the canvas was not initially paint enabled
Add environment variable hook for Subcanvas testing

I have encountered some trouble with subcanvas behavior. This
commit includes a hook that tests the evironment variable called
USE_PRIMA_SUBCANVAS, which can be set on or off by the user on the
command line.
Add subfigure example