Rename repository to image-tracking

[tangobravo]

"Marker" often implies specifically designed fiducials of some sort. Although "Image Markers" is a term I've heard used, most implementations have settled on "Image Anchors" and "Image Tracking" for describing more generalized techniques for detecting and tracking textured 2D images known ahead of time.

This repo has a slightly confused identity itself, as the repo is marker-tracking but the explainer uses image-tracking as the feature name and is focused on providing user-supplied bitmap images to be used for tracking, rather than a particular fiducial design.

The API currently proposed here (and the Chromium implementation) is for tracking general textured images, so I propose renaming this repo to image-tracking would ease confusion. Fiducial-specific tracking algorithms could have different specs and APIs - eg a separate "qrcode-tracking" feature with it's own repo and spec (and privacy concerns). Keeping this one focussed on images might unblock progress on implementations shipping this feature.