Add image support for fltk printing

[Shai Ayal]

Hi

The attached changeset implements image printing using the fltk
backend (and as a bonus fixes a bug in the gl-renderer image support
that prevented display of images with width and/or height of 1). There
is some problem with how the gl2ps library decides if something
obscures an image so sometimes the axis background will obsecure the
image in ps file (i.e. the image won't be shown ...).
I do not know of an easy fix.

Also, images are stored in the ps file as uncompressed 24 bit per
pixel hex strings, so beware of printing large images (e.g. each image
megapixel will take 6MB in the ps file)

Shai

gl2ps_images.changeset
Description: Binary data