Refactor to remove mask-to-polygons dependency

[AdeelH]

🚀 Feature

RV uses https://github.com/azavea/mask-to-polygons for vectorization, but the library is no longer being maintained. We should move the vectorization code into RV itself and remove this dependency.

Motivation

The library is no longer being maintained.

Pitch

We should move the vectorization code into RV itself and remove this dependency.

Alternatives

Alternatively, we can continue to use that library in its current state. But there is at least one possible improvement that can be made to that library: azavea/mask-to-polygons#17.

Additional context

N/A