Fix #6828 break the flyTo end position in some cases

[benoitbzl]

mapbox-gl-js version: 0.48

browser: Chrome

Steps to Trigger Behavior

At the end of animation, when final scale does not produce the correct zoom value, the zoom value is forced to the desired one (purpose of the fix #6828). But the scale value is not modified accordingly, so that the center computation goes wrong at

mapbox-gl-js/src/ui/camera.js

Line 970 in 8dd624d

const newCenter = tr.unproject(from.add(delta.mult(u(s))).mult(scale));

The scale value does not match the one of the transform object, so the unproject() function produces wrong values.

[mourner]

Would you be open to submitting a PR for this?

[benoitbzl]

I am not sure yet how to compute the scale value based on the zoom value, but I will give it a try this afternoon.

[benoitbzl]

I submitted PR #7223.