Source tiles not reloaded when making associated layer visible

[ivovandongen]

In the case where a source has no visible layers and the visibility of one of the layers is set to VISIBLE, the source tiles are not reloaded because the source is disabled when Style::relayout() is called:

        if (source && source->baseImpl->enabled) {
            source->baseImpl->reloadTiles();
        }

The source is only thereafter enabled in Style::recalculate(). This was added to prevent reloading sources that aren't used: 9127299

This results in layers not appearing visibility is set #7225.