Two plugin-rework patches

[Charlie Brej]

Ray Strode wrote:
> On a different note, I think at some point it may make sense to merge
> parts of ply_image and ply_frame_buffer into a new ply_pixel_buffer.

Yeah, I have been thinking about it. It will involve major breakage so I'd like 
to do it right first time. A good idea would be to copy the GDK pixbuf method of 
having reference counting and rowstride. That way it becomes easy to use 
subsections of images while not worrying what happens if the parent image is no 
longer used.