Start adapting to mapnik 'image_data_any' branch

[springmeyer]

• all tests should still pass against Mapnik 2.3.x
• for now skips any image types besides rgba8
• next step: add tests and support for int8, int16, and float32

[springmeyer]

landed in mapnik master/3.x: https://github.com/mapnik/mapnik/pull/2608