Smooth transition between zoom levels and panning by approximating previously cached textures at other levels

Gray tiles (unoccupied) appear on the map while provider plugin is requested to fetch tiles for the current scene when panning or zooming to new map areas that have no cached tiles (texture and/or disk).
An attempt to display approximated textures for new tiles by scaling texture(s) of other tiles cached at different zoom levels. Upon receiving actual PNG data from provider plugin, tiles with approximated texture will be updated.
As a result, a tile can have an approximated visual appearance immediately once panned/zoomed to.
This feature will be toggled by a configuration option for QML Map object, along with the choice of fast or bilinear filtering approximation.