For cross-process layers, I want an API like gfxASurface::CreateSimilarSurface(), except keyed on a gfxASurface "descriptor" rather than an existing gfxASurface. My use case is creating a surface in a child process that's "similar" to one in its parent process. I only really need this for X11 up front, so I'll leave the other implementations TODOs. Joe suggested using a Drawable's XID as the descriptor for X11, which sounds really good modulo potential lifetime issues: the Drawable might after "GetDescriptor()" and before "CreateFromDescriptor()". This isn't a problem for my use case, but I'll see how hard it is to construct a descriptor from other "permanent" Xlib attributes.

We've got all the bits and pieces in place we needed for this from various other bugs.