Add support for runtime styling when snapshotting

[frederoni]

Now that snapshots landed in Android/Core #9748 & iOS/macOS #9891, let's take it one step further by exposing runtime styling to the snapshotters:

On the iOS/macOS side, I'd propose something along these lines:

[snapshotter startWithStyleHandler:^(MGLStyle *style) {
    [style addLayer:polyline];
    [style addSource:source];
} completionHandler:^(UIImage *snapshot, NSError *error) {
    
}];

@ivovandongen @fabian-guerra @tobrun

[1ec5]

On iOS/macOS, would chaining operations to the snapshot’s start require something along the lines of #6386?

[frederoni]

@1ec5 #6386 would probably come in handy. I figured modifying the style later would make more sense when being used in a scroll view instead of allocating a map view and style upon initialization but I don't have any strong opinion about that at this point.

[1ec5]

If this feature is implemented, we’d be able to conditionally enable transparency in snapshots (#10118) based on whether the background style layer is transparent.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[1ec5]

Fixed in #16268. mapbox/mapbox-gl-native-android#220 tracks hooking the feature up on Android. mapbox/mapbox-gl-native-ios#200 tracks hooking the feature up on iOS and macOS.