[Popover] considerations when migrating to `popover` API

[Westbrook]

With the coming implementation of APIs like the popover attribute, it will be possible to have UIs that features Popovers and desendents of Popovers. In doing so, the filter found at:

spectrum-css/components/popover/index.css

Line 70 in cafd1a3

filter: drop-shadow(var(--mod-popover-shadow-horizontal, var(--spectrum-popover-shadow-horizontal)) var(--mod-popover-shadow-vertical, var(--spectrum-popover-shadow-vertical)) var(--mod-popover-shadow-blur, var(--spectrum-popover-shadow-blur)) var(--mod-popover-shadow-color, var(--spectrum-popover-shadow-color)));

will stack as seen here: https://codepen.io/Westbrook/pen/BaqmJjP with the .spectrum-Popover element with the red drop shadow filter. We need something like what is seen in .spectrum-Popover--alt so that subsequent filters can be removed so that they do not stack.