Expose cairo functionality without needing pycairo

[artemp]

The ability to pass Pycairo objects to mapnik is excellent (allows for drawing multiple times to canvas). But difficulty of installation of pycairo and the extra complexity of passing pycairo objects to mapnik rendering and creating cairo contexts/surfaces, highlight the need to be able to write to cairo formats without pycairo using an API as simple as the current method of writing to AGG-based formats.

The attached patch makes it possible to write to cairo formats with only cairo/cairomm installed and using the python api like:

{{{
mapnik.render(map,'output.pdf') # extension will be used to select cairo renderer

or

mapnik.render(map,'output.png','ARGB32') # keyword will be used to select cairo renderer's PNG output (which can be non-Antialiased)
}}}

[artemp]

[springmeyer] patch attached attempts to abstract the rendering to cairo file formats, and exposes the rendering to Cairo image formats (PNG) in non-antialiased mode.

This needs review and more thought:

• How should rendering threads be handled in render_to_file when cairo is used?
• Should this be further abstracted to avoid doing if type == "pdf" in the python bindings...
• Should cairo formats be exposed in 'save_to_string' methods?
• Should the Image object be extended to support/abstract cairo methods?

[artemp]

[springmeyer] going to have a looksie at this before 0.7.0 if I have time

[artemp]

[springmeyer] modified patch applied in 1531 to 0.7 branch.

[artemp]

[springmeyer] pushing to 0.8.0 as we'll need to integrate this into trunk

[artemp]

[springmeyer] looks like I added this to trunk in r1546 and forgot to close.