Add anchor option to Marker

[jfirebaugh]

Fixes #5640.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• manually test the debug page

[mollymerp]

looks good to me! just one comment/suggestion. 🙇‍♀️

[mollymerp]

should we add a check here so that if a user provides something like new mapboxgl.Marker({anchor: 'left'}) it is valid instead of having to use new mapboxgl.Marker(null, {anchor: 'left'})?

if we don't want the former to be valid, we should fire a better error because right now it is just Uncaught TypeError: Cannot read property 'add' of undefined

[jfirebaugh]

Gonna send a separate PR for this in a sec.

[mollymerp]

Hmm actually I think the offset's might be backwards
In these screenshots the red dots are markers with [0, 0] offset and anchor: middle
anchor: left

anchor: right

the docs state:

A string indicating the Marker's location relative to the coordinate set via {@link Marker#setLngLat}.

I would expect the results to be flipped 🤔

[jfirebaugh]

The behavior is chosen for consistency with icon-anchor. The anchor is the part of the popup/marker/icon that's closest to its coordinate.

Though now looking at icon-anchor, I see that it should be 'center' rather than 'middle'.

[mollymerp]

gotcha – makes sense when looking with a popup that has the same anchor setting (blue dot is the one with the top-left anchor):