Transparent PNG export for VTT multi-level maps

When exporting a multi-level map for use in a VTT (e.g., Foundry VTT), each floor's exported image currently includes the full ground/floor texture as an opaque background — even in areas that should be open to the level below, like windows, balconies, and stairwells.

This causes a problem with VTTs that support viewing multiple levels simultaneously (such as Foundry's Levels feature): a token on the ground floor should be visible from an upper floor through a window or balcony opening, but instead it's hidden because the upper floor's exported image opaquely covers that entire area.

Request: Add an export option (PNG, to support alpha transparency) that only renders the floor/roof geometry actually present at that level, leaving everything else — window openings, balconies, stairwells, atriums — transparent. This would let VTTs correctly show lower levels through those openings instead of being blocked by a solid floor texture.