Room center offsets in SVG markup

[hopperelec]

For the tool I made for quickly generating maps, I have a data-label-offset property for moving labels. When I make a proper map creator, I plan on "rendering" these labels client-side anyways, since the <text> elements and their coordinates are quite bloated. This offset should be generalized to define the centre of the room, allowing it to also be used for icon positions, door lines on the door mapper, and for the room selection shortcut defined in #30