Fix deprecation warning, Add getFilters function

[dgarciabriseno]

I added a couple changes.

1. event.image -> event.data. event.image is deprecated and fills the console with logs when accessed a lot.

2. I added a method to retrieve existing filter options, I needed this so I can add filter new filters dynamically instead of overwriting existing filters.

usage:

let filters = viewer.getFilters()
filters.append({ /* new filter details */})
viewer.setFilterOptions({filters: filters})

[Aiosa]

Hi, this repository is no longer maintained and will be superseeded by https://github.com/RationAI/OpenSeadragonFiltering, since the plugin does not conform to the v6 OSD API, which makes possible for the plugin to work with any drawer including WebGL.