Stickers across multiple parts (STAMPs)

[Orion Pobursky]

Relevant part:
https://www.ldraw.org/cgi-bin/ptdetail.c...84ad01.dat

I think we have a gap in our spec. STAMP shortcuts are not allowed per the spec but placement of this sticker on a model is a nontrivial endeavor without a weird origin.

I see 2 solutions:
- Allow STAMP shortcuts
- Allow cases where the sticker may extend past the edge of the single part used.

I think the latter is the least complex and "better" solution.