Fixed plotPixmap  - GridConverter remembers the conversion  - Do not convert all gridpoints if not needed

Summary: Fixed diFIeld::plotPixmap

Test Plan: plot a field as fill_cell with the same projection as the field, and with a different projection.

Reviewers: #diana, alexanderb

Reviewed By: #diana, alexanderb

Differential Revision: https://phab.met.no/D70