Change marker color and/or style

[ejm714]

Currently CyFi explorer uses a red outlined marker to mark the sample point on the imagery (regardless of level). This has a few downsides:

• red suggests high severity
• red is not colorblind friendly

We want this marker to be easy to see (against water, land, or clouds) and not cover up too many relevant pixels. A light grey x that is of heavy enough weight may be a good candidate.