test coverage of raster warping + colorizing

[springmeyer]

Have a hunch our visual test coverage is missing for this scenario (warping and colorizing). When we get to checking/filling this gap we need to check if

mapnik/include/mapnik/renderer_common/process_raster_symbolizer.hpp

Line 150 in 3047b53

if (nodata_) data_out.set(*nodata_);

can be removed, which I recall removing from the non warping case.

[springmeyer]

We've got coveralls support now, so coveralls with keep us honest on this. We'll be generally pushing on more coverage support as part of #2811