Add a function that takes a path, and offsets it
by some distance, applying line-join parameters
as needed.
This commit just adds the api, the implementation
will be in the following commit.
A gskpathop is a pointer to a graphene_point_t* with the low bits used
to encode the GskPathOperation. It's an easy way to introduce API for
operations.
So far it's just used to replace GskStandardOperation.