Update STB library version to 1.16 (#120)

Also remove files not used by svgpp.

Update version

Merge pull request #58 from svgpp/basic-shapes-fix

Fix basic_shapes_policy::collect_attributes handling

Merge pull request #55 from JaimeIvanCervantes/rapidxml-print-clang-fix

rapidxml_ns_print: Added function declarations to solve compilation errors with function visibility on clang/gcc