Allow configuration of JPEG quality

[springmeyer]

Hardcoded at 85 right now here: http://trac.mapnik.org/browser/trunk/src/image_util.cpp#L48

[Dennis]

The following patch resolves this issue. It does not break C++ API or Python bindings.

Right now JPEG files are saved with a line in C++ like: save_to_file<ImageData?32>(buf.data(),str.str(),"jpeg");

Similar to png256 one can add a suffix to "jpeg" specifying the quality between 0 and 100. If the suffix is anything different from that an exception is thrown specifying the problem (either 'out of bounds' or 'not a number'). Not adding a suffix sets quality to 85 as before.

[springmeyer]

Dennis, great, thanks for this addition.

I've marked this as 'design decision needed' so others can take a look and also 'needs documentation', as before we apply we will also need to update the python docstrings here:

http://trac.mapnik.org/browser/trunk/bindings/python/mapnik_python.cpp#L301

[springmeyer]

applied in r1078 and docstring updated in r1082