rtpjpegdepay: fix framerate parsing for locales that use a comma as floating point

rtpjpegdepay: fix framerate parsing for locales that use a comma as floating point

atof() converts strings according to the current locale, but the
framerate string will likely always use a dot as floating point
separator, so use g_ascii_strtod() instead (but also canonicalise
the string before, so we can handle both formats as input).