Consolidate colormap code

In the `visualization` folder, we are using the `colour` library to create a red to green colormap. We should instead just use matplotlib for this, like we do in the dashboard generation code in `gtsfm/evaluation/visualize_benchmark_comparison.py`

`visualization/vis_utils.py` might be a good place for this.

Start in visualization/vis_utils.py and compare the existing red-to-green colormap logic with the matplotlib usage in gtsfm/evaluation/visualize_benchmark_comparison.py. Consolidate the visualization colormap code around matplotlib, remove the colour-library usage from this path, and verify that the affected visualization and dashboard generation still produce the intended colormap.

Written by the indexing model from the issue text.