add flatgeobuf geo data format support

[RandomFractals]

and map view for .fgb files with example in the built-in map gallery

see: https://github.com/bjornharrtell/flatgeobuf

[RandomFractals]

countries.fgb loaded in geo data viewer ...

date file from: https://github.com/bjornharrtell/flatgeobuf/blob/master/test/data/countries.fgb