Reduced threshold for Stereographic projection.

The calculation for the threshold of the `Stereographic` projection was changed in  ae2d0e8 but (probably by mistake) the value of threshold for default globe parameters was increased by an order of magnitude. This causes the extent of stereographic plots to be incorrect.

This PR reduces the threshold so that when default parameters are supplied the projection's threshold will be 1e5 as it was in v0.8.0. Doing so reverts the changes made to expected values in `test_inverted_poly_removed_hole` and `test_inverted_poly_clipped_hole` in ae2d0e8, and also requires some of the vector test images to be regenerated since they use a stereographic projection and the extents were ever so slightly incorrect when they were generated originally.