Symbol layer offset should be relative to geometry when placement is line

[adamal]

Motivation

My specific use case: Drawing arrows on line.

Problem with current functionality

My current implementation is two separate layers. One of type: 'line' the other (with the arrows) of type: 'symbol' with symbol-placement: 'line'

Giving the line layer an offset helps visualize when the track is going both ways. However it seems to be impossible to align the symbol layer with the line layer in this case. The line layer's 'offset' property is a single number relative to the line direction, whereas the symbol layers offset requires an absolute offset (with two values) relative to the map. This also applies symbol-placement: 'line'.
See screenshot of problem below.

Suggested solution

Change the offset behaviour of symbol layers to match line layers when symbol-placement: 'line'