Add 100% zoom (pixel parity)

[JakeSmarter]

This is a followup to https://forum.mapillary.com/t/useless-and-confusing-zoom/7670/5

In short, the current zoom implementation has no controls to zoom‑in to 100%, that is to reach pixel parity between image and display device pixels, where each image pixel matches a display pixel.

There are three natural options to solve this issue:

• Make zoom level 1 zoom‑in to 100%
• Make zoom level 3 zoom‑in to 100%
• Add a 100% zoom‑in button

Mind different projections! However, supporting the linear projection only while not limiting this functionality to linear projection images only should be good enough for now.