(from memory) Upon drawing the skeleton, you need to listen out for the attachment/slotname.once the maskout
name comes through, you need to grab the verts and triangulate. You need to then disable writing to the buffer (inside this mask). The next step is listen out for the maskin
which you want to enable writing to the buffer in. then anything after that (in draw order)will be masked
In spine your draw order should be:
stuff you want to mask
maskin
maskout
stuff drawn behind mask