[plt-scheme] Bugs in PLoT for 299.100 (fixed)

I tracked down the bugs that were giving be a problem in porting my
applications to Version 299.100.  I'm not sure the best way to suggest
patches, but this one is easy if someone can apply them for me.

Both of the bugs are due to problems in the new ffi interface to plplot and
are in the plplot.ss fle of the plot collection.

1)  In pl-setup-page, xleng and yleng should be of type _plint (not _plflt).
That was causing the wrong values to be loaded for the length and width,
which then defaulted to 800 x 600.

2) In pl-fill, take off the initializer '= (length x values)' and change the
type to _plint, i.e. (n : _plint).  This changes the calling sequence to
match what it was previously and prevents my extension - which uses fill to
draw histograms - from failing.

With those changes, my graphics code works and looks like it does in Version
209.

Doug