Update leaflet.wms.js

[AndreaPQ]

Added support for 'zIndex' and 'pane' options to wms.Overlay ( requires 'isBack' options set to null)

[gregallensworth]

Almost works, but with one more change as of October 14, 2020

Add to the wms.Overlay 'options' block these two lines:

        'zIndex': null,
        'pane': null,

Otherwise, these options will be treated as params and not opts down in initialize()

With this second patch, the zIndex and pane options work A-OK, and it would be great to see this merged into the main distribution.

[AndreaPQ]

Almost works, but with one more change as of October 14, 2020

Add to the wms.Overlay 'options' block these two lines:

        'zIndex': null,
        'pane': null,

Otherwise, these options will be treated as params and not opts down in initialize()

With this second patch, the zIndex and pane options work A-OK, and it would be great to see this merged into the main distribution.

added parameters to options block