using constexpr with std::numeric_limits consistently

[springmeyer]

For best performance with C++11 it is now possible to use things like std::numeric_limits<type>::max() as constexpr. At least on mac and unix - windows does not yet support this. In reviewing current usage of std::numeric_limits we are currently:

• Using constexpr std::numeric_limits in one place, and it breaks on windows - pixel_cast.hpp(134): error C2134: 'std::numeric_limits<float>::max': call does not result in a constant expression - refs Remove numeric_cast from image_util.cpp #2893 (comment)
• Using static const std::numeric_limits in several places including src/grid/grid.cpp and include/mapnik/text/font_feature_settings.hpp
• Using non static const in a lot of different places

The plan going forward is to clean this up so that we are using: static MAPNIK_CONSTEXPR std::numeric_limits ... everywhere possible and where MAPNIK_CONSTEXPR will be constexpr on unix and const on Windows.