Issue #1206: Use floating point values in pdb.gimp_vectors_stroke_translate()

This small patch attempts to resolve the API issue noted in #1206 (closed).

gimp_vectors_stroke_translate () is the public facing version of gimp_stroke_translate (), which uses floating point values for its x and y offset parameters. However, gimp_vectors_stroke_translate () defines those parameters as ints, truncating the possible values to whole numbers. This patch converts the parameters to gdoubles instead of gints.

Per #1206 (comment 219047), this API can be updated for 3.0. The only part of GIMP that currently calls this method is one instance in the selection to path plugin, and updating the API did not have a noticeable effect on its output.