Fix plot_gcp.py (#800)
Fix plot_gcp.py (#800)

Summary:
I don't think this script has been updated since the C++ refactoring that brought many Python types/functions to native.

Made some minor fixes to get it to run again. :pray:﻿

Pull Request resolved: https://github.com/mapillary/OpenSfM/pull/800

Reviewed By: fabianschenk

Differential Revision: D31231031

Pulled By: YanNoun

fbshipit-source-id: 331436702772c1817bd727d38841fd953417d904

mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM