Easey-style smooth zoomPanTo + GridLayer fractional zoom support

A work in progress.

- [x] have a basic animation loop through center/zoom values working (without intermediary tiles loading yet)
- [x] make fractional zoom working in GridLayer so that it loads tiles from a round zoom and scales the layer with CSS Transform as it would do when animating
- [ ] fix fractional zoom wrapping/bounding
- [ ] clean up fractional zoom changes
- [ ] implement curved Easey-style zoomPanTo
- [ ] make sure events (zoom/zoomstart/zoomend, move/movestart/moveend) are fired correctly during zoomPanTo