schedulePrefetchGlStencilOsmTilesForCustomMask

Queues GlStencilOsm vector tile fetches for the same z/x/y grid as the encoded weather layer, without consulting built-in land/water/transport mesh caches. Used when masking is driven only by TileLayer.stencilLayerMask (EncodedTileLayer.glStencilMaskKind may be MaskLayerKind.NONE); without this, base-OSM MVTs for custom vector masks (e.g. Demo 3 motorways) load only on demand during draw, which makes panning feel choppy.