frontend: map: allow round shape and faded edges

[ES-Alexander]

Current behaviour

The map is fixed to a rectangle, with sharp edges, so it clearly stands out from the underlying content, which can be undesirable at times.

Expected or desired behaviour

1. Allow changing the map to a round shape (e.g. elliptical or circular)
2. Allow fading the edges of the map with a gradient, so it's less prominent

Example:

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.