backend_pgf: \pgftext now requires \color inside argument (fix #3779) [backport to 1.4.x]

On systems using TeX Live 2014 `\pgftext` doesn't handle text color the same way as before. Add `\color` command inside the text argument to adapt to this new behaviour.
It was redundant because in the end `s` is embedded into `\pgftext[%s]{%s}`.