Increase image comparison tolerance for test_grdimage_global_subset (#3019)

Author: seisman

pygmt/tests/test_grdimage.py

@@ -173,7 +173,7 @@ def test_grdimage_over_dateline(xrgrid):
     return fig
 
 
-@pytest.mark.mpl_image_compare(tolerance=3.0)
+@pytest.mark.mpl_image_compare(tolerance=3.6)
 def test_grdimage_global_subset(grid_360):
     """
     Ensure subsets of grids are plotted correctly on a global map.