clang warning: misc-definitions-in-headers

[MichaelChirico]

This operator definition is triggering a clang warning:

https://github.com/r-lib/svglite/blob/b547e14c68ecf9fb0f6411706d460559ec54d505/src/SvgStream.h#L69-L75

https://clang.llvm.org/extra/clang-tidy/checks/misc-definitions-in-headers.html

I'm not 100% sure the right approach here -- probably to just move the definition inside devSVG.cpp instead of the header?