Fix bug which caused type conversion to get done twice for elements of arrays. This caused arrays of t, nil to always end up a 1s, not 0s and 1s.

Change to compare (png) pictures generated with the pngcairo driver. The comparison is done using ImageMagick.

Some work on trying to get this to be exactly the same as the PLplot example, not completely successful as yet.

Correct to exactly match the equivalent PLplot example. To get an exact match plvpor() coordinates need to be specified with double precision.