export: add option to create poster static map

[laurentS]

I added this functionnality as I needed it myself. It doesn't really address any existing issue, hence this pull request.
I intend to improve upon it, but getting your attention on it would help in orienting my efforts in the right direction.
What it does is add 2 params to the command line export command (--pages_x and --pages_y) to define over how many pages across and down you want your static export broken down. Once printed, the pages can be assembled to produce a poster. It should also take into account #1587 to make more sense.

[springmeyer]

Interesting @laurentS - so are you using the multiple pages approach to keep memory usage down? Mapnik's renderers support offset_x and offset_y parameters that can allow for creating a map once and then rendering from it multiple times to images that represent a subset - this might be a slightly more efficient way to do this.

[laurentS]

Cool! I wasn't aware of the offset_x and offset_y parameters. I'll try to amend my code to use them, it sounds like it would be more efficient.
As for memory usage, I'm not sure what you mean by that. I use multiple pages to produce posters without having to manually adjust each page. I print out the series of files, tape them together, and it makes really nice wallpaper :)
(plus it's really helpful for my humanitarian field work, combined with HDM-cartoCSS rendering, when you have nothing bigger than an A4 printer)

[stevage]

Interesting - just came across this. Is there any more news? I once tried to do something similar, printing across 6 x A3 sheets. I did it manually, and it was a lot of work and very fiddly. An automatic process would be useful.

OTOH, maybe a guide to achieving the same thing using ImageMagick would be more appropriate.

[laurentS]

@stevage thanks for the feedback. The code I wrote actually worked back then, but I haven't touched it in several months. I'd have to merge my changes with the master branch, and probably add a bit of UI around it to make it complete, but the command line was definitely usable.

[mattyb]

+1