Gizmo size changes when translating camera.

**Original report ([archived issue](https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-rendering/issues/48)) by Nate Koenig (Bitbucket: [Nathan Koenig](https://bitbucket.org/%7Bc862cdd9-fcc8-4419-9fc7-e20db14b8fcb%7D/)).**

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# Summary

The gizmo size changes when translating the camera. The Euclidean distance from the camera to the selected visual is used to calculate the size of the gizmo. Instead, the distance from the camera to a plane could be used.

# Motivation

This would be a minor improvement to the gizmo, and is low priority.