Add 'image' curve option: 'pixels'

[zmughal]

See http://www.bersch.net/gnuplot-doc/image.html. This avoids interpolation with certain terminals such as SVG.

Current workaround using deprecated "complex 'with'":

#!/usr/bin/env perl
use feature qw(say);
use PDL;
use PDL::Graphics::Gnuplot;

my $z = rvals(3,3);
my $gp;

$gp = gpwin('svg', output => 'test-no-pixels.svg' );
$gp->plot( { with => 'image' }, $z );
say $gp->{last_plotcmd};

$gp = gpwin('svg', output => 'test-pixels.svg' );
$gp->plot( { with => 'image pixels' }, $z );
say $gp->{last_plotcmd};

test-no-pixels.svg:

test-pixels.svg: