Support retina mode

[maRci002]

Hello, I made some changes to support simulating retina mode (note: If geoserver supports retina @2 tiles then it it advised to use them instead of simulating).

It won't detect if device supports retina because I don't want force user to use Material app because of MediaQuery.of(context).

However it is documented how to use it:

  /// If `true`, it will request four tiles of half the specified size and a
  /// bigger zoom level in place of one to utilize the high resolution.
  ///
  /// If `true` then MapOptions's `maxZoom` should be `maxZoom - 1` since retinaMode
  /// just simulates retina display by playing with `zoomOffset`.
  /// If geoserver supports retina `@2` tiles then it it advised to use them
  /// instead of simulating it (use {r} in the [urlTemplate])
  ///
  /// It is advised to use retinaMode if display supports it, write code like this:
  /// TileLayerOptions(
  ///     retinaMode: true && MediaQuery.of(context).devicePixelRatio > 1.0,
  /// ),
  final bool retinaMode;

Moreover if retina is enabled then MapOptions's maxZoom should be maxZoom - 1, currently MapState doesn't detect any maxZoom based on Layers like Leaflet (If not specified and at least one GridLayer or TileLayer is in the map, the highest of their maxZoom options will be used instead.).

Note: this PR needs #584 so merge this only when it is accepted.

closes #212

[matthiasdittmer]

@maRci002 @HugoHeneault I compared rendering quality of this Flutter lib to the native osmdroid lib a few weeks.

The map looked better and more crisp on osmdroid (same bounding box). Is this related to this density fix?

I traced down the complete widget tree to find the issue (image resize algorithm, ...).

[maRci002]

#584 updated so I merged that into this to avoid conflicts whenever they got approved.