Calculation of `pixel_centre` is invalid

[PeterJCLaw]

Perhaps I'm misunderstanding what the pixel_corners property returns, however it looks like the pixel_centre value makes the assumption that they're the pixel-aligned square bounding box of the marker rather than the pixel coordinates of the N corners of the marker.

Computing the centre of such a bounding box is highly unlikely to be useful to any consumer given that there are no guarantees that it matches the centre of the marker itself.

[PeterJCLaw]

From my testing this afternoon, pixel_corners does indeed contain the actual pixel corners and not a bounding box.