fix for 1373, where bounds doesn't get updated after a mapController …

[ibrierley]

For https://github.com/fleaflet/flutter_map/issues/1373

I think setState gets called, but the onPositionChanged gets called before _bounds is updated (inside build). I've also added an extra check for a valid bounds check to be called on a test new bounds (we don't want to update _bounds if the new position is erm out of bounds).

[JaffaKetchup]

If you're happy with this, I'll just approve it. Or I can test it tomorrow (but I haven't been keeping up with the conversation).

[ibrierley]

Give it a day anyway, in case someone spots a flaw.

[JaffaKetchup]

@ibrierley You happy for this to be merged?

[ibrierley]

Yes, lets update I guess, and fix if anyone screams.