Fix easeTo interpolation on pitched maps

[mourner]

A PR in place of #4320. Should close #4178 and #3112.

Changes easeTo implementation to have more robust interpolation, fixing weird behavior on pitched maps, making it more consistent with flyTo, and makes the definition of offset less ambiguous.

Haven't updated expected values in the camera tests, but otherwise this is ready for review.

cc @joswinter

Launch Checklist

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

[mourner]

This turned out to be more complicated than I thought. We can't simply interpolate linearly through projected center because e.g. as you zoom in to a non-centered point, the scale increases exponentially while map center interpolates linearly, leading to a curved animation.

[mourner]

OK, I think I finally got this right, with proper offset handling, and the tests pass. The easing while zooming interpolation formula is still probably not 100% scientifically correct, but it works well in practice, and me and @anandthakker couldn't come up with anything better.

I'd like to improve the easing code further as there is still some logic duplication going on between easeTo and flyTo, but I'd rather do that in a follow-up PR.

[mourner]

@1ec5 should this be ported to native? It changes the way easeTo animations are calculated quite substantially.

[1ec5]

The native implementation of easeTo diverged from the GL JS implementation shortly after I ported it, but we probably do have to revisit the native implementation in light of mapbox/mapbox-gl-native#5292.