location: Fix gclue_location_set_heading_from_prev_location()
I've been running GeoClue with patches which disable setting of "heading" from compass (which is broken in the Pinephone anyway), but noticed that headings derived from movement were really strange.
I fixed the implementation in gclue_location_set_heading_from_prev_location() and added some clarifying comments.
Here's a comparison of the old and new implementations (I reimplemented these in Python, but I think I got them right). The dlon and dlat are differences between current and previous location coordinates.
| dlon | dlat | Old | New |
|---|---|---|---|
| +0.0 | +2.0 | 90.0000 | 0.0000 |
| +1.0 | +2.0 | 63.4349 | 26.5651 |
| +1.0 | +1.0 | 45.0000 | 45.0000 |
| +2.0 | +1.0 | 26.5651 | 63.4349 |
| +2.0 | +0.0 | 0.0000 | 90.0000 |
| +2.0 | -1.0 | 333.4349 | 116.5651 |
| +1.0 | -1.0 | 315.0000 | 135.0000 |
| +1.0 | -2.0 | 296.5651 | 153.4349 |
| +0.0 | -2.0 | 270.0000 | 180.0000 |
| -1.0 | -2.0 | 243.4349 | 206.5651 |
| -1.0 | -1.0 | 225.0000 | 225.0000 |
| -2.0 | -1.0 | 206.5651 | 243.4349 |
| -2.0 | +0.0 | 180.0000 | 270.0000 |
| -2.0 | +1.0 | 153.4349 | 296.5651 |
| -1.0 | +1.0 | 135.0000 | 315.0000 |
| -1.0 | +2.0 | 116.5651 | 333.4349 |