[WFS] force request on zoom in if the features count match MaxFeatures of the dataSourceUri

On Zoom In, current mGetExtent always contains the new window's extent.
Are we sure that all data has been loaded ? NO.

If the current number of features match the maxFeature parameter, we have to force redefinition of mGetExtent as the new window's extent (smallest).

If the current number of features is lower than the maxFeature parameter, nothing to do : we already have all the data.