Headless snapshots

[zugaldia]

We currently provide snapshot, that allows developers to capture a Bitmap of the map asynchronously on the UI thread with SnapshotReadyCallback.

We hear from developers that it'd be useful if, in addition, the following use cases were supported:

• Ability to generate full snapshots of purely the map style
• Ability to do so without a view hierarchy
• Ability to do so on arbitrary threads

Related ticket on iOS: #1701.

cc: @ivovandongen @tobrun @Guardiola31337

[tobrun]

This happened in #9748, tail work is ticketed out in #9916 and #9914.