extent parameter in EPSG:4326 whatever the map's projection is

[jeanpommier]

In the widget API, one can define the initial map extent using the 'extent' parameter. In case someone uses a map projection different than EPSG:4326 (eg mercator), it would nevertheless be easier to define the extent using geographic coordinates. This proposal reprojects the extent (in geographic) to the map's projection, whatever it is.